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UNIVERSITY  OF  ILLINOIS  BULLETIN 

Vol.  4.  FEBRUARY  16,  1907  No.  12 

[Entered  at   Urbana,   Illinois,  as  second-class  matter] 


STUDIES  FROM  THE  SCHOOL  OF  CERAMICS 

NUMBER  TWO. 


STUDIES  IN  GLAZES. 


PART  I. 


FRITTED  GLAZES. 


BY 


R.  C.  PURDY  AND  H.  B.  FOX. 


PUBLISHED   FORTNIGHTLY   HY  THE  UNIVERSITY 


[Reprinted  from  the  Transactions  of  the  American-  Ceramic  Society, 
Vol.  IX.    Paper  read  at  St.  Louis  meeting,  February,  1D07.] 


FRITTED  GLAZES;  A  STUDY  OF  VARIATIONS  OF  THE 

OXYGEN  RATIO  AND  THE  SILICA-BORAdC 

ACID  MOLECULAR  RATIO. 

BY 

]{<>ss  C.  Pubdy  AND  HARBY  B.  Fox,  Champaign,  Ills. 

Two  systematic  studies  of  raw  lead  glazes,  published 
in  the  Transactions  of  the  American  Ceramic  Society, 
have  done  much  to  indicate  the  limits  of  variation  in  their 
composition  and  heat  treatment. 

Accounts  of  similar  studies  of  fritted  glazes  could  not 
be  found  by  the  writers,  after  a  search  in  the  literature  of 
ceramics.  This  want  of  definite  information  is  certainly 
not  because  fritted  glazes  are  unimportant  or  little  used, 
for,  on  the  contrary,  they  form  the  basis  of  decoration  for 
the  most  costly  wares,  and  are  to  the  white-ware  manufac- 
turer the  glaze  par  excellence  in  the  making  of  non-crazing 
china. 

This  dearth  of  information  regarding  fritted  glazes  is 
no  doubt  due  largely  to  their  complexity  in  composition 
and  the  consequent  difficulties  in  making  the  necessary 
calculations.  When  there  are  but  two  variables  as  in  the 
case  of  raw  lead  glazes,  only  the  simplest  calculations  are 
required  to  formulate  series  in  which  one  or  both  numbers 
vary  in  some  predetermined  ratio,  but  when  three  variable 
factors  are  to  be  considered,  as  in  the  case  of  fritted  glazes, 
the  planning  of  a  like  series  presents  serious  difficulties. 

Mr.  Ashley1,  in  his  able  discussion  of  the  two  papers 
on  the  composition  of  biscuit  bodies  that  appeared  in  Vol- 
ume VII  of  the  Transactions,  demonstrated  clearly  the  in- 
completeness that  is  apt  to  follow  an  attempt  to  determine 
the  limits  of  variation  of  three  factors  when  taken  in  pairs 


Trans.  Am.  Or.  gOC.,  Vol.  VII.   p.   90. 

3 


FRITTED   GLAZES. 


in  six  or  more  wholly  independent  series.  As  a  further 
illustration,  several  series  of  fritted  glaze  studies  were 
formulated,  in  each  of  which  either  the  Al2Os,  Si02  or 
jB203  was  varied  in  an  arithmetical  ratio.  Several  good 
iglazes  were  developed  in  each  series,  and  consequently  it 
i  was  thought  a  very  wide  range  in  the  composition  of  fritted 
glazes  at  certain  temperatures  had  been  determined.  Such, 
however,  did  not  prove  to  be  the  case,  for,  when  the  glazes 
had  been  compared  as  to  their  oxygen  ratio,  Si02 — B203 
ratio,  and  A1203  content,  it  was  found  that  but  a  very  nar- 
row range  in  composition  had  been  used.  As  a  result,  the 
large  amount  of  experimenting  had  practically  been  for 
naught. 

Confident  that  the  difficulties  in  the  case  of  fritted 
glazes  could  be  overcome,  the  senior  writer  gave  consider- 
able thought  to  methods  by  which  the  range  in  the  varia- 
tions of  the  three  factors,  A1203,  Si02,  and  B203  could  be 
determined.  The  one  given  in  the  following  report  seemed 
to  be  the  most  feasible  for  the  purpose.  Many  improve- 
ments in  the  details  of  the  original  plan  were  made  by  the 
junior  writer,  and  it  is  felt  that  the  method  as  here  pre- 
sented is  simple  in  its  detail,  and  permits  a  very  broad 
study  of  the  limits  of  variation  in  fritted  glaze  composition. 

There  are  important  details  of  the  fritted  glaze  prob- 
lem which  cannot  be  determined  by  any  consideration  of 
the  oxygen  ratio,  the  molecular  ratio,  nor  in  some  cases, 
even  the  ultimate  chemical  formulae  of  the  glazes.  As  an 
illustration,  oxygen  ratio  does  not  seem  to  be  a  factor  in 
determining  the  kind  and  quantity  of  the  several  ingre- 
dients that  should  be  incorporated  into  the  fritt,  and  what 
should  be  added  'raw',  in  order  to  obtain  a  given  fusibility, 
coefficient  of  expansion  and  contraction,  fluidity,  solvent 
effect  on  body,  production  of  color  tints,  etc.,  etc.,  conse- 
quently such  questions  are  not  considered  in  the  experi- 
ments here  reported,  although  as  stated  before,  their  im- 
portance is  recognized. 

Owing  to  the  scarcity  of  reliable  scientific  data  on  the 
fritted  glazes  actually  used  in  the  different  ceramic  indus- 


FRITTED   GLAZES.  O 

tries,  but  very  little  idea  of  the  limits  of  variation  in  the 
composition  and  heat  treatment  of  such  glazes  could  be 
obtained. 

The  following  are  a  few  of  the  formulae  studied: 

White  Ware  Glaze,  E.  Mayer,  Trans.  Am.  Cer.  Soc,  Vol.  I,  p.  57. 

0.064  K20    ^  ro  ai     ft.n 

0.192  Na2C>!  Ain  JJ'81     Sl°2 

SSSSJ         i«»      c— 4 

Earthen-ware  Glaze,  Dr.  H.  Hecht,  Thonindustrie-Zeitung,  1897, 
quoted  by  Ashley,  Trans.  Am.  Cer.  Soc,  Vol.  VII,  p.  92. 
0.10  KoO     1  f2.1  SiOo 

0.30  CaO      }  0.2  AI2O3  4 
0.60  PbO     j  10.4  B20;!  Cone  010 

White  Ware  Glaze,  K.  Langenbeck,  p.  122,  Chemistry  of  Pottery. 
1 1. 25  KNaCn  f3.0  Si02 

0.50  CaO      }  0.3  A1203  < 
0.25  PbO     J  10.5  BsOj  Cone  04 

WaU  Tile  Glaze,  R.  C.  Puxdy,  private  notes. 

0$?N?ol  fl.835Si02 

S    CaO      W2  ^ 

Wall  Tile  Glaze,  R.  C  Purdy,  Trans.  Am.  Cer.  Soc,  Vol.  VII,  p.  81. 

on  CaO      °-19  A1*0^ 

o'lgpbo  l0.57B2O.,  Conel 

GENERAL    PLAN    OF    INVESTIGATION. 

A  study  of  these  successful  commercial  fritted  glazes 
revealed  the  fact  that  there  were  four  factors  to  be  con- 
sidered in  the  make-up  of  the  chemical  formulae. 

1st.  The  character  of  R.  O.  i.  e.  the  kind  and  equiva- 
lent amounts  of  each  present. 

2nd.  The  oxygen  ratio,  i.  e.  the  ratio  of  the  total  oxy- 
gen in  the  acids  to  the  total  oxygen  in  the  bases. 

3rd.     The  molecular  ratio  of  the  silica  to  boracic  acid. 

4th.     The  equivalent  content  of  alumina. 

It  is  quite  obvious  that  since  there  are  oxides  of  seven 
or  eight  different  basic  elements  used  for  different  purposes 
in  fritted  glazes,  there  are  a  great  many  possible  BO  com- 
binations that  could  be  and  should  be  tried  if  the  studv  of 


6  FRITTED   GLAZES. 

the  entire  field  of  fritted  glazes  is  to  be  attempted.  Since 
for  each  110  there  is  a  large  number  of  possible  variations 
in  the  other  factors,  it  is  evident  that  it  would  be  imprac- 
tical to  include  all  possible  combinations  in  one  investiga- 
tion. It  was  decided,  therefore,  to  limit  this  study  to  one 
arbitrarily  chosen  RO. 

The  RO.  The  following  was  taken  because  it  repre- 
sents the  average  character  of  the  RO  used  in  white-ware 
and  wall-tile  glazes. 

0.126  Na2Q 
0.124  KzO 
0.500  CaO 
0.250  PbO 

The  Oxygen  Ratio.  In  order  to  not  only  cover  the 
range  of  variation  in  the  oxygen  ratio  of  the  commercial 
fritted  glazes  studied  in  the  preliminary  survey  of  the  sub- 
ject, but  also  to  determine  if  possible  the  practical  limits 
of  such  variation,  it  was  thought  best  to  go  beyond  Avhat 
was  taken  to  be  the  extreme  minimum  and  maximum.  Since 
the  most  workable  oxygen  ratio  in  a  raw  lead  glaze  has 
been  found  to  be  t'^2,  and  since  one  of  the  fundamental 
objects  in  the  use  of  fritted  glazes  is  to  secure  increased 
acidity  and  consequent  reduction  of  the  coefficient  of  ex- 
pansion and  contraction,  this  ratio  (1  :  2  )  was  adopted  as 
the  minimum.  At  the  time  the  study  here  reported  was 
made,  it  was  thought  that  the  oxygen  ratio  of  1. :  4  was  the 
maximum  in  use,  and  therefore  a  ratio  that  would  repre- 
sent a  step  beyond  this  limit  was  chosen  as  the  maximum. 

During  the  progress  of  the  investigation  it  was  found 
that  the  maximum  limit  possible  under  all  conditions  had 
not  been  chosen,  for,  not  only,  were  there  good  glazes  de- 
veloped with  this  oxygen  ratio,  but  it  has  since  been  found 
that  Seger  had  used  leadless  barium  fritted  glazes  that 
ranged  as  high  as  1 :  (>  with  reported  success. 


FRITTED   OI.AZK9. 


The  following  are  the  oxygen  ratios  adopted 


Oxygen  in 

Oxygen  In 

Total  Bases 

Total  Acids 

2.00 

2  60 

3.00 

3.50 

3  75 

4.00 

4.50 

The  Silica-Boracic  Acid  Molecular  Ratio.  It  was  in- 
ferred from  a  study  of  commercial  fritted  glazes  that  the 
silica-boracic  acid  molecular  ratios  varied  from  1  : 0  to 
1 : 0.20,  and  therefore  1 :  0.25  was  chosen  as  the  maximum. 
Here  also  it  has  since  been  learned  that  the  possible  maxi 
mum  ratio  was  not  adopted,  for  Edwards  and  Wilson1 
report  the  successful  use  of  a  much  higher  ratio. 

The  silica-boracic  acid  ratios  adopted   in  this  study 
were : 


0.25       0.20 


1 
0.11 


1 

0.13 


1 

0  09 


1 

0  05 


0.01 


and 


The  Range  in  Alumina  Content.  It  is  quite  obvious 
to  those  who  have  made  a  study  of  glazes  in  general,  that 
the  permissible  maximum  equivalent  of  A120:.  is  dependent 
upon  the  temperature  at  which  the  glaze  is  designed  to 
mature,  and  since  cone  10  had  been  arbitrarily  chosen  as 
the  maximum  temperature  at  which  to  burn  the  glazes  of 
this  study,  it  was  decided  that  0.45  equivalent  would  be  a 
larger  equivalent  than  would  be  used  commercially  in  the 
vast  majority  of  instances.  It  is  common  experience  that 
a  small  equivalent  of  Al2Oa  is  quite  accessary  to  the  devel- 
opment of  an  insoluble  or  stable  glaze,  and  thai  as  a  rule 
white  ware  glazes  contain  from  0.25  to  0.40  equivalents.    It 


'Trans.  Eng.  C.  S.  1904-5,  p.  24. 


FRITTED   GLAZES. 


was  thought,  therefore,  that  0.1  equivalents  could,  with 
justice  to  the  study,  be  considered  as  the  minimum  amount 
feasible. 

The  equivalent  molecular  variations  of  A1203  chosen 
were  0.1;  0.15;  0.20;  0.25;  0.30;  0.35;  0.40  and  0.45. 

Shown  in  tabular  form  these  three  variable  factors  can 
be  represented  as  follows : 


Division  into  Groups  by 
Oxygen  Ratio 

Division  of  Groups  into  Series 

by  the  Molecular  Ratio  of 

SIO,  :  B203 

Division  of  Series  into  Members 

Differentiated  by  their  Ala03 

content 

Group  No. 

O.  R. 

Series  No. 

Ratio 

Member 
Designation 

A1,03  Equiv. 

I 

n 

ni 

IV 

V 

VI 

VII 

2.00 
2.50 
3.00 
3.50 
3.75 
4-00 
4.50 

1 

2 
3 
4 
5 

6 

7 
8 

1 
1 
1 
1 
1 
1 
1 
1 

0.25 
0.20 
0.17 
0.13 
0.09 
0.05 
0.01 
0.00 

: 

c 
d 
e 
f 

g 
h 

0.10 
0.15 
0.20 
0.25 
0.30 
0.35 
0.40 
0.45 

CALCULATION  OF  THE  GLAZES. 

The  calculation  of  the  proportions  in  which  the  var- 
ious extremes  in  these  series  are  to  be  blended  to  produce 
any  given  glaze  of  the  448  provided  for  in  the  above  scheme 
is  somewhat  perplexing,  and  as  before  stated,  was  made 
the  subject  of  a  good  deal  of  thought.  The  following  method 
of  calculation  was  finally  found  to  be  the  most  satisfactory. 
Considerable  more  space  is  given  therefore  to  the  detailed 
explanation  of  the  method  than  would  perhaps  be  justifi- 
able in  ordinary  cases. 

First  Method :  Formula  of  Glazes.  The  oxygen  ratio 
of  a  fritted  glaze  can  be  expressed  by  the  formula 

2y+3z 

<A>  °-K-  =  3iTr 

in  which  O.  R.  stands  for  oxygen  ratio;  y  represents  the 
molecular  equivalent  of  Si02  present;  z  the  molecular 
equivalent  of  boracic  acid;  x  the  molecular  equivalent  of 
alumina;  and  the  numeral  1,  the  oxygen  in  the  RO  when 
the  total  equivalent  of  RO  is  reduced  to  unity.     In  the 


FRITTED   GLAZES. 


9 


statement  of  conditions  for  any  one  glaze,  the  oxygen  ratio 
and  alumina  equivalent  are  given,  i.  e.  known.  Each  for- 
mula thus  contains  only  y  and  z  as  unknowns.  Since  the 
ratio  of  y  to  z  is  also  given  in  the  siliea-boracic  acid  ratio, 
we  have  the  equation 

( B )       y  :  z  :  :  1  :  b  or  z=by. 
With  these  two  simultaneous  equations  y  and  z  are  readily 
determined. 

Since  the  A1203  equivalent  varies  regularly  in  each 
series,  it  is  quite  obvious  that  the  Si02  and  B203  content 
of  the  several  glazes  in  each  series  differs  by  a  constant 
factor  that  can  be  obtained  by  subtracting  the  Si02  and 
B203  in  the  first  member  (a)  from  the  Si02,  and  B2Ox 
equivalent  in  the  second  (b),  so  that  only  the  first  two 
members  of  the  series  need  be  calculated  by  equations  (A) 
and  (B). 

Second  Method.  It  was  developed  in  the  actual  carry- 
ing out  of  the  calculations  by  the  first  method  that  only 
the  first  two  terms  of  the  series  in  the  end  groups  need  be 
calculated  by  the  above  equations.  After  obtaining  the 
Si02  and  B203  for  the  members  of  each  series  in  Groups 
I  and  VII  as  given  in  the  first  method,  the  corresponding 
serial  members  of  the  intermediate  groups,  it  was  found, 
could  be  obtained  by  a  blending  calculation  on  the  basis  of 
the  difference  in  the  oxygen  ratio  and  the  rule  of  extreme 
and  mean  differences.  The  blending  factors  for  the 
groups  are 


Parts  of  Group  I 

Parts  of  Group  VII 

Group       I 

1.0 

0.0 

n 

0.8 

0.2 

m 

0.6 

0.4 

IV 

0.4 

0.6 

v 

0.3 

0.7 

VI 

0.2 

0.8 

u    vn 

0.0 

1.0 

Equiv.  Si02  for  S  1,  Q  1  "a"  (.9464)  times  0.8  =  0.75632 
"  '«        "  SI,  GVII"  a"  (2  1  "3)  times  0.2  =  0.42547 

"  Sl,GII"a"  then      0.75632  +  0.42546  or  1.1818 


10  FRITTED   GLAZES- 

By  this  method  the  Si02  and  B203  content  of  the 
glazes  in  all  the  groups  can  be  very  readily  obtained  by  the 
use  of  simple  factors  as  above  shown. 

Blending  of  the  Glazes. 

There  are  many  ways  in  which  the  glazes  indicated 
in  the  above  table  could  be  blended,  but  it  was  decided  that 
this  could  be  done  most  readily  and  with  the  greatest  ac- 
curacy when  the  number  of  "original"  or  weighed  glaze 
batches  was  the  smallest,  so  that  the  fritts  might  be  melted 
in  large  quantities  at  one  time,  and  thus  errors  in  weighing 
fractional  quantities  made  as  small  as  possible.  Each 
individual  glaze  can  then  be  made  by  weighing  portions 
of  these  few  made-up  glazes  as  explained  later. 

The  minimum  number  of  "weighed"  glazes  that  could 
be  used  in  this  blending  scheme  was  found  to  be  eight,  as 
follows : 


1. 

Group       I. 

Series 

1 

member  a 

2. 

Group       I. 

*' 

1 

h 

3. 

Group      I. 

it 

8 

"         a 

4. 

Group      I. 

" 

8 

h 

5. 

Group  VII. 

•• 

1 

a 

6. 

Group  VII. 

" 

1 

h 

7. 

Group  VH. 

" 

8 

a 

8. 

Group  VII. 

c< 

8 

h 

Of  these  eight  glazes  the  first  four  are  from  Group  I 
and  the  last  four  are  from  Group  VII.  In  the  first  group, 
therefore,  the  maximum  amount  of  each  of  the  first  set  of 
four  glazes  will  be  used  and  in  Group  VII  the  maximum 
amount  of  each  of  the  last  set  of  four  glazes.  The  amounts 
of  the  glazes  in  each  set  that  are  used  in  the  intervening 
groups  are  in  the  same  proportion  as  the  oxygen  ratio  of 
those  several  groups.  By  again  noting  the  extreme  and 
consecutive  variations  in  oxygen  ratios  in  the  tabulated 
statement  of  the  groups  and  series,  it  will  be  seen  that 
these  proportional  amounts  for  each  group  as  noted  above 
are  as  follows: 


FRITTED 

GLAZES. 

Group       I 

1.0 

0.0 

Group     II 

0.8 

0.2 

Group   III 

0.6 

0.4 

Group   IV 

0.4 

0.6 

Group     V 

0.3 

0.7 

Group    VI 

0.2 

0.8 

Group  VII 

0.0 

1.0 

Total, 

3.3 

37 

11 


That  is,  in  the  first  group,  10-33  of  the  total  amount 
of  the  first  four  weighed  glazes  will  be  used ;  in  the  second 
group  8-33  of  the  first  four  and  2-37  of  the  second  four,  etc. 

Having  thus  calculated  the  distribution  of  the  eight 
weighed  glazes  in  the  several  groups,  the  next  step  is  ob- 
viously the  determination  of  the  proportional  amount  of 
each  of  the  "weighed"  glazes  in  each  series. 

Since  the  "weighed"  glazes  represent  series  1  and  8 
of  each  group,  by  making  blending  calculations  on  the 
basis  of  the  silica  content,  the  intermediate  series  of  each 
group  are  made  up  of  proportional  parts  of  the  extreme 
:*Tiefl  as  follows : 


Serie. 

Weighed  Glazes  of 

Weighed  Glazes  of 

1st  Series 

8th  Series 

1 

1.0000 

0.0000 

2 

0.8461 

0.1539 

3 

0.7451 

0.2549 

4 

0  5984 

0.4016 

'» 

0.4363 

0.5637 

6 

0  2558 

0.7442 

T 

00542 

0.9458 

8 

0  0000 

1.0000 

Total 

3-9359 

4.0(541 

To  illustrate  how  this  proportional  distribution  of 
the  weighed  glazes  is  made  in  the  separate  groups  take 
Group  II,  Series  4.  It  was  determined  that  for  Group  II 
there  should  be  used  8-33rds  of  the  total  amount  of  the 
weighed  glazes  belonging  to  Group  I  and  2-37ths  of  the 


12 


FRITTED  GLAZES. 


weighed  glazes  belonging  to  Group  VII. 
Group  II,  therefore,  there  would  be  used : 

/0J5984      _8\     f  weighed  glaze  j  and  2. 
V3.9359       33/ 

(Hix^)°,wei8hedgIaK!6a,,d6- 

1  of  weighed  glaze  3  and  4. 
1    of  weighed  glaze  7  and  8. 


In  Series  4,  of 


3.9359 
0.4016  8 
4.0641  X  33 
0.4016  _2_ 
4.0641  X  37 


Similar  calculations  were  made  for  each  series  in  each 
of  the  groups. 

The  sum  or  mixture  of  the  proportional  parts  of  the 
total  amount  of  "weighed"  glazes  1,  3,  5,  and  7,  as  shown 
above,  would  constitute  the  first  member  or  "a"  of  Series  4, 
Group  II  and  the  mixture  of  the  proportional  parts  of  the 
total  amount  of  "weighed"  glazes  2,  4,  6,  and  8,  as  above 
shown,  would  constitute  the  last  member  or  "h"  of  Serie* 
4,  Group  II. 

Having  thus  the  proportional  part  of  the  total  amount 
of  the  "weighed"  glazes  in  the  first  and  last  members  of 
each  series  in  each  of  the  groups,  the  further  distribution 
of  the  "weighed"  glazes  in  the  intermediate  members  of 
each  series  must  be  made  on  the  blending  proportions  as 
obtained  on  the  basis  of  the  difference  between  the  alumina 
content  of  each  member.  These  proportional  factors  are 
found  to  be  as  follows : 


Proportion  of  "a" 

Proportion  of  "  b" 

Members  of  Series 

in  each  Member 

in  each  Member 

a 

1.0000 

0.0000 

b 

0.8571 

0.1429 

c 

0.7142 

0.2858 

d 

0.5714 

0.4286 

e 

0.4286 

0.5714 

d 

0.2858 

0.7142 

f 

0.1429 

0.8571 

g 

0.0000 

1.0000 

Total 

4.0000 

4.0000 

FRITTED   GI>AZES. 


13 


Series  4  Group  II  then  may  be  made  up  by  blending 
the  eight  "weighed"  glazes  in  the  proportional  amount  of 
the  total  of  each  of  the  "weighed"  glazes  as  in  the  table  on 
page  106-107. 

To  demonstrate  that  the  factors  there  developed  give 
true  results,  the  chemical  formula  of  glaze  d,  Series  4, 
Group  II,  is  calculated  as  follows : 


0.00526  Eqv.  glaze  No.  1  contains 


0.00394 
0.00341 
0.00266 
0.00117 
0.00088 
0.00076 
0.00057 


No.  2 
No.  3 
No.  4 
No.  6 
No.  6 
No.  7 
No.  8 


Total 


0.0052 
0.0039 
0.0034 
0.0026 
0.0011 
0.0008 
0.0007 
0.0005 


Al„Oa 


0.0181 


0.0005 
0.0018 
0.0003 
0.0012 
0.0001 
0.0003 
0.0001 
0.0002 


0.0046 


SiO, 


0.0049 
0.0067 
0.0044 
0.0060 
0.0026 
0.0034 
0.0022 
0.0030 


B.O. 


0.0012 
0.0016 


0.0006 
0.0008 


0.0331   0.0042 


Multiplying  these  totals  through  by  an  amount  which 
will  bring  the  RO  to  unity,  or  55.24,  we  have  the  following 
result : 

(1.8284  Si02 
0 .  9998  RO,  0 .  2486  A1S08     ■? 

(0.2820  B20, 

of  which  the  oxygen  ratio  is  1:2.49  + 

Since  the  sum  of  the  elements  brought  in  by  the  re- 
spective amounts  of  the  various  weighed  glazes  employed 
makes  a  glaze  whose  formula  satisfies  all  the  conditions  for 
glaze  'd',  Series  4,  Group  II,  the  above  method  of  calcula- 
tion must  be  correct  in  principle. 

In  the  same  manner,  the  equivalent  amounts  of  the 
eight  "weighed"  glazes  required  for  a  mixture  or  blend, 
having  the  chemical  composition  of  each  of  the  448  glazes 
required  for  the  whole  investigation  can  be  calculated  and 
tabulated. 

These  equivalents  or  factors  are  parts  by  molecules 
but  not  by  weight.  In  the  calculation  of  the  required  parts 
by  weight  of  the  eight  "weighed"  glazes,  the  total  amount 
of  each  glaze  required,  and  the  difference  in  the  combining 


14 


PKITTKD   GLAZES. 

Calculation  for  Compounding 


0.5984 
3.9359 
0  4016 
4.0641 
0  5984 
8  9369 
0  4016 
4  0641 


X 


X 


X- 


x- 


8 

33 

8 

33 

2 

37 

2 

37 


"a"  X 
"h"  X 


0.8571 
4.000 
0.1429 
4.000 


"a"  X 
"h"  X 


0.7142 
4.0000 
0.2858 
4.000 


"a"  X 
"h»"  X 


0.6714 
4.000 
0.4286 
4.000 


"a"  X 
"h"X 


0.4286 
4.000 
0.5714 
4  000 


"a"  X 
"h"X 


0.2858 
4.000 
0.7142 
4.000 


"a"  X- 


h"  X 


0.1429 
4 .  000 
0.8571 
4.0D0 


0  5984 
3.9359 
0.4016 
4.0641 
0.4016 
4.0641 
0.5984 
3.9369 


X 


X- 


X- 


8 
33 
2 
37 
8 
33 
2 
37 


Proportion  of 

Weighed  Glaze 

Number  1 


0  0368 


0  0079 


0  00657 


0  00526 


0  00394 


0.00263 


0  001315 


Proportion  of 

Weighed  Glaze 

Number  2 


0.001375 


0.00263 


0.00394 


0.00526 


0.00657 


0.007885 


0  0368 


Proportion  of 

Weighed  Glaze 

Number  3 


0.0239 


0  00512 


0.004267 


0.003414 


0  00256 


0  00181 


0.000854 


KRJTTKD   GLAZES. 


16 


Scries  4,  Group  II. 


Proportion  of 

Weighed  Glaze 

Number  4 

Proportion  of 

Weighed  Glaze 

Number  5 

Proportion  of 

Weighed  Glaze 

Number  8 

Proportion  of 

Weighed   Glaze 

Number  7 

Proportion  of 

Weighed  Glaze 

Number  8 

0  00822 

0.00536 

0.00176 

0  001146 

0.000864 

0  000294 

0.000191 

0.00147 

0.00096 

0.00181 

0  000587 

0  000382 

0.001174 

0.00076 

0.00256 

0.00088 

0.000573 

0  00088 

0  000573 

0  003414 

0.001174 

0.00076 

0  000587 

0.000382 

0.004267 

0.00147 

0.00(96 

0.00612 

0.000294 

0.00176 

0.000191 

i 

0.001146 

0  0289 

0  00822 

1 

i 

0.00685 

1 

18  FRITTED   GLAZES. 

or  batch  weights  of  each  of  the  "weighed"  glazes  must  be 
considered.  This  latter  consideration  has  not  until  lately 
been  noted  in  the  blending  of  series  of  glazes  or  bodies  and, 
so  far  as  the  writers  can  learn,  it  was  first  suggested  and 
made  use  of  by  the  senior  writer,  in  class  exercises  in  ce- 
ramics at  the  Ohio  State  University  in  1903,  and  was  first 
mentioned  in  published  papers  by  E.  Ogden,  who  was  a 
student  at  Ohio  at  that  time.1  Mr.  Ogden  has  amply  set 
forth  the  necessity  of  noting  the  difference  in  the  batch 
weights  of  the  extremes,  so  that  further  discussion  of  this 
point  at  this  time  is  superfluous.  The  importance  of  tak- 
ing cognizance  of  the  differences  in  combining  weights  of 
the  several  glazes  to  be  blended  is  illustrated  in  the  fol- 
lowing : 

Calculation  of  required  total  amount  of  "weighed" 
glazes  for  entire  experiment.  By  trial  it  was  found  that 
about  60  grams  of  fritted  glaze  was  necessary  to  make  a 
coating  one-sixteenth  to  one-eighth  inch  thick  on  five  3  inch 
by  1  inch  by  1^2  inch  wall  tiles.  It  is  obvious  that  the 
maximum  quantity  of  any  one  of  the  "weighed"  glazes  in  a 
given  blend  would  be  required  in  the  case  where  the 
"weighed"  glaze  is  used  alone,  or  unblended  with  any  of 
the  others.  Considering  this  as  a  safe  criterion  by  which 
the  total  amount  of  each  the  "weighed"  glazes  required  in 
the  entire  series  of  blends  can  be  estimated,  it  is  quite  ob- 
vious that  since,  in  the  case  of  "weighed"  glaze  No.  1,  for 
illustration,  as  shown  in  the  development  of  the  blending 
factors,  10-33  of  the  total  amount  is  required  for  the  first 
1 

group;  and  of  the  quantity  required  for  the  first 

3.9359 
group  is  used  in  the  first  series ;  and  14  of  that  used  in  the 
first  series  is  required  in  the  first  member  or  'a',  it  follows 

10         1          1            10 
that  — X X —  or of   the   total   amount    of 

33     3.9359      4         519.54 


'Trans.  A.  C.  S.,  Vol.  VII,  p.  378. 


FRITTKJ)   GLAZES. 


17 


"weighed"  glaze  No.  1  used  in  the  entire  system  of  blends 

10 

must  be  equal  to  60  grams.       Therefore  60  -4-  or 

519.54 
3117.24  grams  is  the  total  amount  of  "weighed"  glaze  No.  1 
required. 

Having  calculated  the  batch  weight  of  the  eight 
'•weighed"  glazes,  as  shown  later,  it  was  found  that  the 
combining  weight  of  ''weighed"  glaze  No.  1  was  213.78. 

Since  3117.24  was  so  nearly  15  times  the  combining 
weight  of  "weighed"  glaze  No.  1,  15  was  adopted  as  a  fac- 
tor by  which  the  combining  weights  of  each  of  the  eight 
"weighed  glazes  should  be  multiplied  to  ascertain  the  total 
amount  of  each  required  for  blending  the  entire  number  of 
glazes  in  this  experiment,  as  shown  iu  the  following  table : 


Weighed  Glazes 

Combining  Weight 

Factor 

Amount  Required 

1 

213.78 

15 

3207 

2 

296.78 

15 

4452 

3 

206.52 

15 

3097 

4 

305.34 

15 

4580 

5 

293.00 

15 

4395 

6 

462.32 

15 

6935 

7 

303.97 

15 

4560 

8 

481.47 

15 

7222 

Means  of  Minimizing  the  Number  of  Weighings  in 
Above  Blending  Scheme.  By  simple  calculations  it  was 
found  that  if  the  above  scheme  of  blending  was  carried  out 
in  detail  as  given,  2976  separate  weighings  would  be  re- 
quired. By  first  weighing  up  the  "weighed"  glazes  pro- 
portionally into  groups,  then  thoroughly  mixing  the 
blended  glazes  by  passing  them  through  a  60  mesh  sieve  six 
or  seven  times;  taking  these  blended  glazes,  now  16  in  num- 
ber, and  blending  them  proportionally  into  series,  and  then 
finally  into  the  separate  members,  in  other  words,  by  mak- 
ing the  blends  in  three  stages,  there  would  be  required  only 
656  or  less  than  Vi  of  the  number  of  weighings  that  would 
be  required  if  the  amount  of  each  of  the  "weighed"  glazes 


•/ 


FRITTED   GLAZES. 


requisite  for  the  proper  blending  of  each  member,  was 
weighed  out  direct.    This  was  accordingly  done. 

THE   FRITTS. 

Opinion  and  custom  differs  widely  as  to  what  parts  of 
the  glaze  should  be  incorporated  in  the  fritted  portion.  As 
a  rule,  there  is  no  cognizance  taken  of  the  solubility  of  the 
resultant  fritt,  nor  any  attempt  to  harmonize  its  chemical 
composition  with  that  of  the  glaze.  Since  there  is  neither 
law  nor  general  custom  in  this  matter,  the  following  rules 
were  arbitrarily  chosen : 

1st.  The  fritt  should  constitute  at  least  50%  by 
weight  of  the  glaze. 

2nd.  Its  oxygen  ratio  should  be  the  same  as  that  of 
the  whole  glaze. 

3d.  The  fritt  should  contain  (1)  all  of  the  alkaline 
salts  including  the  feldspar;  (2)  all  of  the  boracic  acid 
and  borax ;  (3)  all  but  0.05  equivalent  of  the  required  clay; 
(4)  all  but  0.10  equivalent  of  the  total  calcium  oxide;  (5) 
all  of  the  free  aluminum  oxide;  (6)  only  sufficient  silica 
to  maintain  the  required  oxygen  ratio.  This  left  to  be 
added  raw  to  each  of  the  "weighed"  glazes  the  following : 

0 .  25  Eqv.  white  lead 
0.10  Eqv.  whiting 
0.05  Eqv.  china  clay 
X       Eqv.  flint 

The  fritts  were  made  in  a  crucible  fritt  furnace1 
fired  by  gas.  The  fritts  when  fused  dropped  from  the 
crucible  into  cold  water  for  granulation.  It  was  found 
that  all  but  two  of  the  fritts  were  reasonably  insoluble  in 
water,  but  the  comparatively  ready  solubility  of  the  fritts 
belonging  to  weighed  glazes  No.  3  and  7  developed  a  belief 
that  the  fritts  should  not  be  run  into  water  but  rather  out 


'Manufactured  and  donated  to  the  Ceramic  Department,  Univer- 
sity of  Illinois,  by  W.  D.  Gates,  American  Terra  Cotta  and  Ceramio 
Company,  Chicago,  Illinois. 


FRITTED   GI.AZKS.  19 

onto  a  told  slab,  refritted  and  then  ground  dry.  Mr.  J.  F. 
Krehbiel  suggested  this  plan,  and  has  carried  it  out  with 
good  effect  in  the  crystaline  glaze  experiments. 

With  the  exception  of  the  solubility  of  the  two  fritts 
cited,  all  eight  fritts  seemed  to  be  normal.  Their  fluidity 
was  sufficient  to  permit  free  flowage  through  the  orifice  of 
the  crucible,  and  in  no  case  was  excessive  heat  or  time  re- 
quired to  affect  the  complete  melting  of  the  fritt. 

Materials  Used. 

The  following  materials  were  used  in  this  series  of 
experiments : 

O.  P.  Sodium  Carbonate    NajCO.-? 

C.  P.  Potassium  Nitrate    KNO3 

Whiting  CaCOs 

White  Lead  Pb(OH)2  ZPbCOs 

Flint  SiOs 

Borax  NajB^O?  10H2O 

Boracic  Acid  (Flaky)         B»08  3H20 

Calcined  Aluminum  Oxide  Al 2O3 

Soda  Feldspar.    The  analysis  was  as  foUows : 

Per  cents. 

SiOs 69.36 

AljOs 17.00 

Fes03 0.63 

CaO 0.62 

MgO 0.88 

KsO 6.31 

NaiO 4.79 

The  molecular  formula  of  the  above  is : 

of  which  the  combining  weight  is  669 . 6. 

Potash  Feldspar,  from   Brandywine   Summit,  Pennsylvania.     The 
analysis  was  as  follows : 

Per  cents. 

SiOa.   68.60 

AI2O3 18.40 

FeaOs 0.54 

CaO 0.30 

MgO 0.14 

KzO 10.52 

NaaO 2.00 

Moist 0.17 

l\  A  I   -. 


20  FRITTED  GLAZES. 


The  molecular  formula  of  which  is : 

0.7314  K20   )n™iln, 
0.2109  Na*0  1  11<9  M2°*  \  -   .-  «,.<. 
0.0349  CaO    f  n  n.9  ~    n    f  747  blL>? 
0.0228  MgOJ0-042re2°3j 

of  which  the  combining  weight  is  663.79. 
Georgia  Kaolin.     The  analysis  was  as  follows : 

Per  cents. 

Si02 44.02 

AI2O3 39.51 

Fe203 1.09 

CaO 0.36 

MgO 0-12 

KNaO 0.23 

H20 14.60 

The  molecular  formula  of  the  above  is : 

KNaO  0.0075^|  I.OOAI2O3      ^   1.89  SiOa 

OaO      0.0165  }  V 

MgO    0.0077  J  0.0175  Fe203  J  2.09  H20 

of  which  the  combining  weight  is  257.78. 


PREPARATION  OF  THE  GLAZES. 

The  fritts  were  wet-ground  to  pass  freely  a  100  mesh 
sieve,  dried  in  newly-made  plaster  evaporating  molds,  and 
finally  dried  thoroughly  by  subjection  to  artificial  heat. 

As  no  water  was  poured  off  and  thrown  away,  the 
only  portion  of  the  soluble  materials  in  the  fritts  that  was 
lost,  was  the  trifle  that  entered  the  pores  of  the  plaster 
evaporating  molds.  The  uncertainty  as  to  the  absolute 
constitution  of  the  fritts  when  finally  ready  for  use  was 
the  only  known  irregularity  in  the  whole  experiment,  and 
the  writers  believe  that  this  is  not  very  serious. 

The  eight  "weighed"  glazes  were  then  weighed,  wet- 
ground  for  a  half  hour,  passed  through  a  100  mesh  sieve, 
and  dried  in  plaster  evaporating  molds. 


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22 


KRITTKD   GLAZKS.  23 

From  the  foregoing  tables  it  is  readily  seen  that  the 
difference  in  composition  and  batch  formulae  lies  wholly 
in  the  constitution  of  the  fritt,  except  that  each  group  con- 
tains progressively  more  and  more  free  flint.  For  each 
group,  however,  there  is  a  definite  equivalent  of  free  silica. 
The  equal  equivalent  of  fritt  used  in  each  case  insures  uni- 
formity in  the  make-up  of  the  several  glazes,  so  that  the 
difference  in  behavior  of  the  glazes,  one  from  another,  can 
be  said  to  lie  wholly  in  their  chemical  constitution. 

The  actual  formulae  of  the  glazes,  giving  the  A120:5, 
Si02  and  B203  content  of  each,  has  been  prepared  in  tabu- 
lar form,  but  in  order  to  facilitate  close  comparison  be- 
tween the  composition  of  the  glazes  and  their  results  on 
firing,  the  tables,  marked  Group  I,  Group  II,  etc.,  appear 
in  connection  with  the  results,  instead  of  at  this  place. 

BODY  USED. 

The  body  used  in  these  experiments  was  furnished  by 
the  U.  S.  Encaustic  Tile  Co.,  of  Indianapolis,  Ind.,  through 
the  courtesy  of  Mr.  E.  M.  Ogle.  It  was  delivered  to  the 
laboratories  of  the  Ceramic  Department  of  the  University 
of  Illinois  in  the  shape  of  normally  burned  biscuit  wall  tile 
of  apparently  uniform  density.  The  composition  of  the 
body  as  given  by  Mr.  Ogle  is  as  follows: 

Ball  Clay 40 

China  Clay 40 

Cornwall  Stone ..20 

Flint 22.5 

122.6 
PREPARATION  OF  THE  TRIAL  PIECES. 

The  question  of  how  thick  a  layer  of  the  glazes  should 
be  placed  on  the  tile  was  considered  seriously,  for  it  was 
doubted  if  the  difference  between  the  behavior  of  the  sev- 
eral glazes  would  be  sufficiently  exaggerated  or  emphasized 
if  applied  as  thin  as  is  the  practice  of  the  china  and  white 
ware  potters,  or  even  the  wall  tile  manufacturers.  Between 


24  FRITTED  GLAZES. 

one-sixteenth  and  one-eighth  of  an  inch  was  finally  adopted 
as  the  thickness.  The  glazes  that  stand  as  "good"'  at  this 
thickness  will  surely  stand  well  when  applied  thinner,  and 
those  glazes  which  would  have  a  tendency  to  craze  or  shiver 
would  have  that  tendency  increased  by  increased  thickness, 
and  thereby  display  their  peculiarities  almost  at  once  after 
drawing  from  the  kiln. 

Five  tile,  marked  in  pencil  with  the  proper  group,  ser- 
ies and  member  symbols,  were  thoroughly  saturated  with 
distilled  water,  placed  side  by  side  and  the  glaze  paste  ap- 
plied over  all  five  tile  at  once,  with  a  spatula.  After  dry- 
ing, the  tiles  were  separated,  fettled,  and  re-marked  with  a 
cobalt  stain. 


PLACING  OF  THE  TRIAL  PIECES. 

The  tiles  were  placed  in  the  tile  setters  of  such  capacity 
that  each  held  one  whole  series.  The  members  of  each  ser- 
ies were  placed  in  a  setter  in  regular  order,  so  that  in  case 
any  of  the  tile  should  be  stuck  to  the  bottom  of  the  setter, 
as  a  consequence  of  the  running  off  of  a  portion  of  the  ex- 
cessively thick  glaze,  as  happened  in  a  few  cases,  each  mem- 
ber of  the  series  could  be  readily  identified  by  its  position 
in  the  setter.  The  identification  of  one  specimen  in  each 
setter  was,  therefore,  sufficient  for  the  identification  of  the 
remaining  members  of  the  series. 

For  nearly  all  burns  above  cone  010,  the  tile  were 
placed  on  small  wads  with  sufficient  space  between  the  tile 
and  the  setter  bottom  to  permit  of  considerable  running  of 
the  glaze  without  seriously  cementing  the  tile  to  the  setter. 

BURNING  OF  THE  GLAZES. 

The  glazes  were  burned  in  a  side  down-draft  kiln  de- 
signed by  the  senior  writer,  and  built  for  the  Ceramic  Lab- 
oratory of  the  University  of  Illinois,  as  shown  in  Plate  I. 


K KITTED   GLAZES. 


26 


• 1 I     >  t 


26  FRITTED  GLAZES. 

This  kiln  was  fired  with  coke,  on  the  following  time  sche- 
dule: 

Cone  010  in  12  hours,  1  hour  soaking,  rapid  cooling. 
Cone  05  in  14  hours,  1  hour  soaking,  rapid  cooling. 
Cone  1  in  16  hours,  1  hour  soaking,  rapid  cooling. 
Cone  5  in  18  hours,  1  hour  soaking,  rapid  cooling. 
Cone    10  in  18  to  24  hours,  1  hour  soaking,  rapid  cooling. 

These  heats  were  easily  attained  in  the  time  allotted, 
with  a  very  thin  (3  inches)  bed  of  live  coals  and  an  average 
of  20  to  30  minute  firing  periods.  In  fact,  in  all  but  the 
cone  10  burns,  the  raising  of  the  heat  was  intentionally 
checked,  so  as  to  insure  as  close  approximation  to  the  above 
temperature  schedule  as  possible. 

In  one  of  the  cone  010  burns  and  one  of  the  05  burns, 
the  kiln  was  "smoked"  in  the  early  part  of  the  burn,  which 
caused  a  deposition  of  carbon  in  the  glazes  that  colored 
them  black.  This,  however,  did  not  injure  the  character  or 
reduce  the  value  of  the  results,  for  fortunately  the  smoked 
glazes  were  not  in  the  series  that  matured  at  these  tem- 
peratures. 

Twenty-eight  series  or  one-half  of  the  entire  448  glazes 
were  burned  at  a  time,  thus  necessitating  ten  burns  in  all. 
The  setters  were  placed  in  four  bungs.  In  the  first  few 
burns,  cones  were  placed  near  the  top  and  bottom  of  each 
bung.  In  as  much  as  the  cones  burned  down  equally  in  all 
eight  positions,  a  fact  that  checked  similar  experience  in 
this  kiln  in  several  previous  burns  on  other  forms  of  ware, 
sufficient  confidence  in  the  absolutely  equal  distribution  of 
heat  in  all  parts  of  the  firing  chamber  was  established,  so 
that  in  the  later  burns  only  two  and  sometimes  one  set  of 
cones  was  used  in  a  burn. 

RESULTS  OF  THE  VARIOUS  BURNS. 

Group  I. 

A  table  of  the  compositions  of  the  64  glazes  composing 
this  group  follows  on  page  27. 


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28  F KITTED   GLAZES. 

Fired  at  Cone  010. 

Series  1.     All  fused  to  glasses;  none  attacked  the  body. 
Members  a,  b,  c,  devi trifled  and  badly  crazed  in  both 
thin  and  thick  places. 

Member  d  slightly  devitrified,  crazed,  good  gloss  on 
most  of  surface. 

Members  e,  g,  f,  h,  good  glazes  where  thin,  but  have 
small  pin  holes  on  surface  where  thick.  Pin-holing 
increases  as  B203  decreases.  Crazed  whether  thick  or 
thin. 

Series  2,  3,  4.  All  devitrified  and  all  effloresced.  Devitri- 
fication decreases  with  increase  of  A1203. 

Series  5,  6,  7.  Are  like  2,  3  and  4  except  that  with  low 
A1203  crackling  or  separation  of  the  glaze  begins  in 
series  5  and  increases  progressively  until  it  is  the  most 
manifest  in  series  8. 

Series  8.  Is  very  badly  crackled,  the  glaze  patches  exhib- 
iting a  vitreous  sheen. 

Summary  of  Group  I  at  Cone  010. 

(1)  The  Eqv.  content  of  A1203  with  which  the  best  glazes 
are  developed  at  this  temperature  ranges  from  0.30  to 
0.40,  with  the  ratio  of  Si02  to  B203  1 :0.25. 

(2)  Devitrification  decreases  with  increase  of  A1203  and 
the  decrease  of  B2Os. 

(3)  Efflorescence  was  general  in  whole  series. 

Fired  at  Cone  05. 

Series  1.  Devitrification  is  less  at  this  temperature  than 
at  cone  010.  Crazing  is  finer  meshed  in  the  glazes 
which  have  lower  equivalent  of  A1203.  Crazing  more 
pronounced  in  all  glazes  at  this  temperature  than  at 
010. 

Series  2,  3.    All  glazes  of  these  two  series  are  dimmed  less 
at  this  temperature  than  they  were  at  010. 
The  most  fusible  member  of  both  series  is  "e,"  having 
an  A1203  content  of  0.30  Eqv.     In  both  series,  it  has 
but  a  trace  of  dimness. 


FRITTED  GLAZES.  29 

The  members  f,  g,  h,  in  both  series  show  graded  in- 
crease in  refractoriness. 

Series  4,  5,  t>.  These  three  series  exhibit  a  most  peculiar 
appearance  in  that  they  have  passed  from  what  ap- 
peared to  be  devitrification  in  the  cone  010  burn,  to  a 
blistered  dull  surface  due  to  the  boiling  that  precedes 
quiet  fusion. 

Members  e  and  f  seem  to  be  the  most  fusible  of  these 
series. 

Series  7.  Members  a,  b,  c,  present  same  crackled  appear- 
ance as  at  cone  010. 

Members  d,  e,  f,  are  devitrified,  matt-like  in  appear- 
ance; e  being  a  beautiful  matt,  but  badly  crazed. 

Series  8.  Carbonized  badly,  but  are  apparently  more 
fused  than  at  cone  010. 

Summary  of  Group  I  at  Cone  05. 

(1 


(2 

(3 
(4 

(5 

(6 

(7 

(8 


Good  glazes  having  an  A1203  equivalent  of  0.3  to  0.4 
occur  in  Series  1,  2  and  3. 

Devitrification  decreases  with  increase  of  A1203  and 
decrease  of  B203. 

None  effloresced. 

Fair  matt  surface  occurs  in  Series  7  with  0.30  Eqv. 
A1203. 

Crazing  is  more  pronounced  in  cone  05  burn  than 
at  010. 

Glaze  e,  series  1,  has  the  longest  range  so  far  devel- 
oped, being  good  at  010  and  at  05. 

Glazes  f  and  g  are  very  nearly  matured  at  010  and 
fully  so  at  05. 

(razing  decreases  with  increase  of  A1203  and  changes 
from  fine  mesh  to  long  hair  lines. 

Fired  at  Cone  1. 


Series  1 .  Members  a,  b,  c,  d  have  good  gloss ;  are  perfectly 
matured ;  crazed  in  fine  meshes  and  have  eaten  into  the 
body,  glaze  "a"  being  the  worst  in  this  respect. 


30  FRITTED  GLAZES. 

Members  e,  f,  g  are  good,  well  matured  glazes;  "e"  is 
considerably  crazed  but  the  craze  lines  are  long  and 
some  distance  apart ;  "f"  is  less  crazed  and  "g"  has  but 
one  craze  line. 

Member  g  is  not  quite  matured. 

What  is  stated  in  the  discussion  of  the  05  burn  in  re- 
ference to  the  decrease  in  devitrification  phenomena  of  ser- 
ies 1,  seems  to  find  confirmation  in  the  cone  1  burn,  for  the 
glazes  that  were  badly  devitrified  at  010  were  less  so  at  05, 
and  not  at  all  at  cone  1,  but  at  cone  1  the  edges  of  the  tile 
are  eaten  away  by  the  glaze,  and  this  eating  away  of  the 
edges  decreases  as  the  glaze  increases  in  A1203.  This  lends 
support  to  the  doctrine  (1)  that  A1203  counteracts  devitri- 
fication; (2)  that  glazes  having  the  acidity  of  this  group 
must  contain  at  least  0.30  Eqv.  of  A1203  to  satisfy  the  acid 
content. 

This  series  brings  out  another  very  significant  fact 
that  has  been  noted  in  many  other  isolated  examples,  to- 
wit:  that  when  the  glaze  is  compelled  to  feed  upon  the 
body  to  gain  the  A1203  required  to  make  a  perfect  glaze, 
fine  mesh  crazing  is  sure  to  follow.  This  increase  in  craz- 
ing with  increase  of  A1203  does  not  accord  with  Seger's 
law  in  which  the  introduction  of  bases  of  high  molecular 
weight  is  credited  with  power  to  decrease  crazing.  But,  as 
will  be  noted  later,  Seger's  law  in  regard  to  decrease  of 
crazing  with  increase  of  A1203  does  hold  true  when  the 
original  content  of  A1203  of  the  glaze  is  considered.  In- 
crease in  A1203  by  eating  into  the  body  increases  rather 
than  decreases  crazing. 

No  scumming  or  devitrification  was  apparent  in  any 
member  of  this  series. 

Series  2,  3  and  4.  The  members  of  these  series  resemble 
series  1  in  every  respect  save  that  of  maturity.  In 
series  1,  all  members  but  h  are  fully  matured,  while 
in  series  2,  3  and  4  g  is  likewise  unmatured. 
The  fine-mesh  crazing  in  a,  b,  c,  d,  and  to  some  extent 
in   e,   is  shown  in  these  series  as  in  series  1,  but  the 


KBITTED   GLAZES.  31 

members  f,  g,  h,  are  freer  from  crazing  as  the  1^0.; 
decreases,  i.  e.  progressively  from  series  1  to  scries  4 
inclusive.  This  would  seem  to  be  rather  significant 
in  view  of  the  fact  that  all  the  succeeding  series  have 
dimmed  surfaces  which  resemble  devitrification  more 
than  immaturity. 

Series  5  and  6.  Not  a  single  member  of  these  series  is 
matured. 

Members  a,  b,  c,  d,  and  e  show  progressive  decrease  in 
dimness  (probably  devitrification)  with  increase  of 
A1203.    None  have  eaten  into  the  body. 

Series  6.  Members  a  and  b  are  smooth,  devitritied  and 
fine-mesh  crazed. 

Other  members  are  in  the  "boiling"  stage.  Member  d 
with  0.25  A1203  exhibits  this  more  than  the  others. 

Series  7.  All  members  of  this  series  cover  the  tile  perfectly 
showing  that  increase  of  heat  treatment  from  cone  05 
to  1  has  been  sufficient  to  cause  these  glazes  to  flow 
enough  to  pass  from  a  erackeled  condition  to  a  perfect 
coating  of  glass. 

Series  8.  All  members  are  dim  with  slight  evidence  of  devi- 
trification. 
All  are  crazed  in  fairly  fine  meshes. 

Summary  of  Group  I  at  Cone  1. 

(1)  Good  Glasses  developed  at  this  heat  are  as  follows: 
Series  1     a  to  h  inclusive. 

Series  2  a  to  g  inclusive. 
Series  3  a  to  g  inclusive. 
Series  4     a  to  g  inclusive. 

(2)  Good  glazes,  having  only  hair  line  crazes,  or  none  ;it 
all  and  being  free  from  pinholes. 

Series  1  e,  f,  g,  h. 

Series  2  e,  f,  g. 

Series  3  f,  g. 

Series  4  f,  g. 


32  FRITTED   GLAZES. 

(3)  Glazes  d,  e,  f,  series  7,  that  exhibited  good  matt  text- 
ure at  cone  05  were  "boiling''  at  cone  1,  showing  that 
at  cone  05  their  matt  surface  was  due  entirely  to  im- 
maturity. 

(4)  At  this  temperature,  eating  into  the  body  decreases 
with  decrease  in  B20;!  and  increase  in  A120;;. 

(5)  Crazing  passes  from  fine  mesh  to  total  absence  with 
increase  to  A1203.  With  low  A1203  (0.1  to  0.25  and  0.8 
inclusive)  decrease  in  B203  does  not  seem  to  affect  the 
character  or  amount  of  crazing,  but  with  higher  A1203 
there  is  shown  a  progressive  decrease  in  crazing  with 
decrease  of  B203.  This  would  indicate  at  least  four 
important  facts. 

(a)  Increase  in  A1203  decreases  crazing. 

(b)  Increase  of  Si02,  retaining  constant  oxygen 
ratio,  likewise  decreases  crazing,  but 

(c)  A1203  is  a  more  powerful  factor  than  Si02 
in  checking  crazing  at  this  heat  treatment 
and  oxygen  ratio. 

(d)  The  facts  noted  in  a  and  c  only  hold  true 
when  the  original  Al2Oa  content  is  consid- 
ered. When  the  glaze  is  compelled  to  borrow 
A1203  from  the  body,  a  strain  is  established 
that  increases  crazing  in  proportion  to  the  ex- 
tent to  which  the  glaze  has  attacked  the  body. 

On  looking  down  into  the  thicker  portions  of  the  glazes 
which  have  eaten  into  the  body,  and  which  exhibit  this  fine 
mesh  crazing  to  the  greatest  degree,  there  appears  to  be  a 
separation  or  splitting  between  portions  of  the  glazes  next 
to  the  body  and  those  nearer  the  surface.  It  is  quite  evi- 
dent from  this,  that  the  portion  of  glaze  in  contact  with  the 
body  is  of  a  different  composition  from  that  near  the  sur- 
face. There  cannot  be  ready  diffusion  of  materials  in 
glazes  of  this  oxygen  ratio,  even  when  relatively  high  in 
B203.  From  the  fact  that  the  glazes  lowest  in  A1203 
have  eaten  into  the  body  most,  exhibit  fine-mesh  crazing  to 


FBITTBD   GLAZES.  33 

the  greatest  degree,  and  show  a  separation  be!  ween  the  por- 
tion of  glaze  contiguous  to  the  body  and  that  above,  it  is 
concluded  that  this  fine-mesh  crazing  is  due  more  largely 
to  extraction  of  A1203  than  to  the  extraction  of  Si02  from 
the  body,  which  owing  to  its  viscosity,  diffuses  very  re- 
luctantly, and  further,  that  this  fine-mesh  crazing  increases 
with  the  increase  of  A1203  so  obtained  by  the  glaze. 

(6)  Devitrification  extends  only  from  series  5  to  8  in- 
clusive, or  over  a  proportional  range  of  Si02  B203  from 
1 :  0.9  to  1 :  0. 

(7)  It  is  indeed  a  most  surprising  fact  that  with  the 
oxygen  ratio  of  2,  which  is  best  suited  to  the  development 
of  good  glossy  raw-lead  glazes  free  from  boracic  acid,  there 
6hould  not  be  developed  a  good  glass,  when  a  portion  of 
the  glaze  is  fritted,  until  the  ratio  of  silica  to  boracic  acid 
has  been  raised  to  at  least  1 : 0.13  and  then  only  within  a 
very  narrow  range  of  variation  in  A1203.  Indeed,  as  will 
be  seen  in  the  study  of  this  same  group  at  cone  5,  fritted 
glazes  having  an  oxygen  ratio  of  2  have  a  heat  range  that 
is  limited  to  but  a  slight  variation  from  cone  1  until  the 
ratio  of  Si02  to  B203  has  reached  1 :  0.2.  Even  at  the 
Si02 — B203  ratio  of  1 :  2.0  at  least  cone  1  is  required  to  ma- 
ture the  glaze,  and  at  cone  5  it  has  withstood  its  maximum 
heat  treatment. 

On  the  other  hand,  when  the  boracic  acid  has  been  in- 
creased until  the  ratio  of  Si02  to  B203  stands  at  1 :  0.25 
there  seems  to  be  established  a  degree  of  fusibility  and  a 
restraint  against  devitrification  that  permits  of  heat  treat- 
ment ranging  from  cone  010  to  at  least  cone  5  inclusive, 
provided  that  A1203  content  originally  incorporated  in  the 
glaze  is  at  least  equal  to  0.3  or  0.4  equivalents. 

Fired  at  ('one  5. 
Scries  1,  2,  3  and  4.     All  glazes  are  crazed,  fine-mesh  craz- 
ing the  most  pronounced  with  low  ALO..  and  increas- 
ing BoO;.    Glazes  that  were  not  crazed  at  Cone  1  are 
crazed  at  Cone  5  in  long  hair  lines. 


34  FRITTED   GLAZES.  ' 

Except  for  crazing,  the  following  are  good  glazes: 

Series  1     d,  e,  f,  g,  h. 

Series  2     c,  d,  e,  f,  g,  h. 

Series  3     none. 

Series  4  f,  g,  h. 
In  series  1,  fine-niesh  crazing  is  about  as  it  was  in 
same  series  at  Cone  1,  except  in  case  of  member  d, 
which  has  flowed  and  run  off  the  tile,  leaving  only  a 
comparatively  thin  coating  of  glaze.  While  the  body 
shows  evidence  of  having  been  attacked  to  some  extent, 
the  glaze  is  coarser  crazed  and  freer  from  horizontal 
crazing  than  member  e,  which  is  thicker.  This  fact 
suggests  three  things. 

(1)  Fine-mesh  crazing  can  be  decreased  by  de- 
crease in  thickness  of  glaze,  thus  permitting  equal 
diffusion  of  the  A1203  obtained  from  the  body  through- 
out the  whole  mass. 

(2)  The  body  will  be  attacked  less  the  thinner 
the  glaze. 

(3)  That  fine-mesh  crazing  is  due  almost  en- 
tirely to  the  unequal  coefficient  of  expansion  and  con- 
traction of  the  upper  and  lower  portion  of  the  glaze 
layer. 

In  all  of  the  series  of  this  group,  the  fine-mesh  crazing 
is  exhibited  in  glazes  which  at  cone  05  were  either  free 
from  crazing  or  were  crazed  only  in  hair  lines.  Members 
g  and  h  in  all  series  are  still  free  from  this  fine-mesh 
crazing,  but  it  is  evident  that  as  the  heat  increases  in  in- 
tensity, even  though  not  in  length  of  time,  the  glazes 
originally  higher  in  A120;!  are  beginning  to  attack  the  body, 
causing  tension  between  the  upper  and  lower  portions  of 
the  glaze,  that  causes  either  actual  or  incipient  horizontal 
as  well  as  vertical  crazing  and  as  a  consequence,  fine- 
mesh  phenomena ;  and  that  in  the  glazes  originally  low  in 
Alo03  the  alumina  incorporated  from  the  body  is  much 
more  thoroughly  diffused,  causing,  as  a  r- on  sequence,  a  de- 


FRITTED    GLAZES.  36 

crease  in  the  fine-mesh  crazing  over  that  shown  with  less 
intense  heat.  These  facts  are  very  clearly  shown  at  cone 
10,  where  the  glazes  having  the  finest  mesh  crazing  were 
originally  highest  in  AUO3  and  at  lower  heats,  in  some 
cases,  are  entirely  free  from  crazing. 

Series  5,  6,  7  and  8.  Devitrification  is  now  shown  only  in 
members  a,  b  and  c  of  series  6  and  7,  and  in  all 
members  of  series  8.  None  are  free  from  crazing. 
Members  having  highest  original  content  of  A1203  ex- 
hibit pin-holing.  No  glazes  of  promise  shown  in  any 
portion  of  these  series. 

Summary  of  Group  I  at  Cone  5  will  be  included  with 
the  summary  of  the  facts  deduced  from  the  Cone  10  burn. 

Fired  at  Cone  10. 

All  members  of  every  series  of  Group  I  at  this  heat 
treatment  are  crazed  in  fine  meshes. 

In  series  1,  member  a  is  not  so  very  finely 
crazed,  but  the  fine  craze  meshes  increase  regularly 
from  a  up  to  f,  which  had  originally  0.35  Eqv. 
ALO.j  and  then  decrease  slightly  from  member  f 
to'h. 

In  series  2,  member  f  again  marks  the  point  of 
maximum  fiue-mesh  crazing. 

From  series  2  to  8,  the  area  of  mazimum  fine-mesh 
crazing  increases  until  in  series  8,  every  member  is 
crazed  in  exceedingly  fine  meshes. 

Every  glaze  ate  into  the  body  considerably,  those 
showing  maximum  fine-mesh  crazing  being  no  worse 
in  this  respect  than  those  showing  this  feature  in  a 
less  degree. 

Summary  of  Group  I  at  Cones  5  and  10. 

(1)  It  is  quite  evident  that  for  fritted  glazes  of  this 
oxygen  ratio  and  RO,  on  this  body,  cone  5  is  beyond  the 

p,  A  P.— 8. 


36  FBITTED   GLAZES. 

maximum  limit  of  temperature,  for  all  the  glazes  are  more 
or  less  crazed  in  fine  meshes.  On  a  body  that  would  not 
give  up  any  of  its  constituent  parts  to  the  glaze,  or  if  the 
glazes  were  dipped  as  thin  as  is  the  practice  in  the  white 
ware  industry,  many  of  these  glazes  may  have  a  heat  range 
that  would  include  at  least  cone  5  if  not  cone  10.  The  ex- 
treme thickness  at  which  these  glazes  were  applied,  per- 
mitted the  formation  of  two  strata,  the  one  next  to  the 
body  containing  without  a  doubt  additional  A1203,  while 
the  upper  strata  was  not  altered  materially  in  composition, 
except  by  the  normal  volatilization  of  B203  and  alkalies. 

Conclusion  mi  Group  I. 

1.  The  good  glazes  developed  at  the  several  tem- 
peratures are  noted  in  the  following  table.  The  glazes 
which  have  a  question  mark  beside  them  are  good  glasses, 
which  might  have  been  good  glazes  if  they  had  been  ap- 
plied thin  enough  to  prevent  the  formation  of  two  strata 
in  the  glaze  layer,  thus  causing  fine-mesh  crazing.  Normal 
crazing  is  not  taken  into  account  in  designating  a  glaze 
as  good. 


Series    Cone  010 

Cone  05 

Cone  1 

Cone  5 

Cone  10 

1     efgh 

d?e?fgh 

b?c?d?efgh 

b? 

c?d?e?fgh 

(atoh)? 

2 

e?f 

b?c?d?efg 

b? 

c?d?e?f?g? 

(a  toh)? 

3 

ef 

b?c?d?efg 

(a  tog)? 

(atoh)? 

4 

c?  d?  e  f  g 

(a  to  g)? 

(b  to  h)? 

5 

f?g? 

(b  to  c)? 

6 

f?g? 

none 

7 

e?  f  ?  g? 

none 

8 

(btof)? 

2.  Series  8  of  this  group  demonstrates  the  fact  which 
was  stated  by  the  senior  writer,  in  1904 ;  viz.,  a  fritted  glaze 
must  have  a  higher  oxygen  ratio  than  1 :  2  or  that  normally 
used  in  raw  lead  glazes.  True,  good  glazes  with  a  fair 
temperature  range  were  developed  in  this  group,  but  they 
required  the  maximum  content  of  B203. 


FRITTKD   GLAZES.  37 

3.  Fine-mesh  crazing  appears  to  be  due  to  unequal 
dissemination  of  constituents  taken  from  the  body,  and  is 
more  pronounced  at  the  low  temperatures  in  those  glazes 
that  are  lowest  in  Al2Oa  irrespective  of  their  B203  content, 
and  as  the  intensity  of  the  heat  increases,  fine-mesh  craz- 
ing decreases  in  the  glazes  low  in  A1203,  in  consequence 
of  the  compounds  extracted  from  the  body,  and  progresses 
steadily  with  increase  in  intensity  of  heat  until  the  glazes 
having  an  A1203  content  of  0.3  to  0.4  Eqv.  that  were  per- 
fect at  the  lower  heat  treatment  because  crazed  in  fine 
meshes  at  the  higher  heat  treatment. 

4.  Devitrification  decreases  as  the  A1203  increases, 
either  as  originally  added  or  taken  from  the  body. 

Group  II. 

A  table  of  the  compositions  of  the  64  glazes  composing 
this  group  follows  on  page  38. 


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FRITTED   GLAZES.  89 

Fired  at  Cone  010. 

Series  1-8  1.  All  were  heavily  impregnated  with  carbon, 
so  that  but  little  can  be  said  of  the  behavior  of  the 
glazes  of  this  group  at  010. 

2.     Members  d,  e  and  f,  of  series  1,  2,  3  and  4,  seem  to 
be  the  most  fusible. 

Fired  at  Cone  05. 

(1)  Members  d  and  e  of  series  1,  were  the  only  two 
good  glazes  developed  with  this  heat  treatment  and 
thickness  of  glaze. 

The  entire  group  had  evidently  been  subjected,  in 
burning,  to  the  influence  of  carbon,  shortly  after  mem- 
bers d  and  e  had  been  fused  into  perfect  glasses,  for 
the  less  fusible  glazes  show  either  an  undulating  or  a 
pimply  surface,  where  they  are  thick,  on  account  of 
the  expulsion  of  CO  or  C02  generated  by  the  com- 
bustion of  the  carbon,  but  were  smooth,  well  developed 
glazes  where  thin. 

(2)  The  glazes  retaining  the  carbon  to  the  end  of  the 
burn  are  to  the  left  of  a  line  drawn  diagonally  from 
member  a  in  series  1  to  member  h  in  series  8.  This 
according  to  Seger1  demonstrated  that  the  glazes 
to  the  right  of  this  diagonal  line  were  nearly  formed 
into  glasses  at  the  time  that  the  kiln  was  ''smoked.'* 
Further,  those  glazes  which  were  fairly  well  devel- 
oped at  cone  010  and  are  good  at  cone  1,  show  least 
of  this  undulating  and  pimply  surface. 

From  these  facts  at  least  three  conclusions  can   be 
drawn. 

(a)  The  most  fusible  mixture  in  series  1,  2  and  3 
is  that  with  0.30  Eqv.  of  A1203. 

(b)  In  series  4,  the  most  fusible  mixture  contains 
0.35  Eqv.  of  A1203 ;  in  series  5  and  6,  0.45  Eqv.  of  A1203. 


'Collected  Writings  of  Herman  A.   Seger.     Amer.  Cer.  Soc.  trans. 
Vol.  II,  p.  592. 


40  FRITTED   GLAZES. 

(3)  Judging  from  the  appearance  of  the  glaze  where  thin, 
and  the  relative  degree  of  maturity  at  cone  010  and  cone  1, 
the  following  glazes  would  have  been  well  developed  had 
they  not  been  "smoked." 

Series  1     d,  e,  f,  g,  h  ? 

Series  2    c,?d,?e,  f,  g? 

Series  3     c?d?e,  f,  g? 

Series  4     e?  f  ? 

Series  5     0. 
Dimness  of  surface  is  equally  pronounced  in   this 
group  when  either  Al2Os  is  low  and  B203  high,  or  A1203 
is  high  and  B203  low. 

Fired  at  Cone  1. 

(1)  The  more  refractory  glazes  of  the  first  four  series 
exhibited  surface  pinholing  or  pitting,  especially 
where  the  glaze  is  thick.  The  surface  pinholing  could 
readily  be  taken  as  indicating  an  over-burned  condi- 
tion, but  such  could  not  be  the  case,  for  they  show  no 
indication  of  being  over-burned  at  cone  5. 

(2)  Members  of  series  4,  5  and  6  are  likewise  pinholed, 
but  owing  to  their  being  less  fusible  than  the  members 
of  the  first  three  series,  most  of  the  trial  pieces  appear 
to  have  a  narrow  border  of  normally  fused  glaze  sur- 
rounding a  more  boiled  and  pitted  patch  in  the  center. 

(3)  Series  7  and  8  are  slightly  blackened  by  carbon, 
showing  that  the  whole  group  had  been  smoked. 
These  three  facts  suggest  that : 

(a)  Pinholing,  which  appears  at  times  when  every 
condition  seems  to  be  normal,  may  be  traced  largely  to 
the  carbon  which  was  entrapped  when  the  glaze  was  almost 
matured,  the  combustion  of  which  produced  gas  that  devel- 
oped blisters,  or  blibs,  which  finally  bursted,  forming  small 
pits  or  pin  holes.  These  pinholes  differ  somewhat  from  the 
blisters  due  to  over-burning,  in  that  the  latter  extend 
much  deeper  into  the  glaze  layer,  and  are  frequently  much 
larger  in  diameter. 


KKITTED   GLAZKS.  41 

(b)  It  cannot  as  yet  be  determined  whether  the  car- 
bon reduced  the  lead,  thus  altering  its  chemical  activity  in 
respect  to  the  boro-silicate  formation  that  is  taking  place, 
or  whether  the  presence  of  carbon  and  the  consequent  car- 
bonic gases  make  the  glaze  more  viscous.  There  is  evi- 
dence in  this  group  that  might  be  taken  to  substantiate 
either  claim.  The  essential  fact  is  that  if  one  portion  of  a 
glaze  has  been  smoked,  and  another  portion  not  smoked,  the 
former  will  have  every  appearance  of  being  over-fired  save 
that  of  the  nature  of  the  pinholes,  while  the  portion  not 
smoked  may  be  a  normally  developed  glaze. 
(4)  The  glazes  of  group  II,  which  either  are  good,  or 
would  have  developed  into  good  glazes  at  cone  1  were 
it  not  for  their  having  been  smoked,  are  as  follows : 


Series  1 

c,  d,  e, 

f, 

g> 

h. 

Series  2 

c,  d,  e, 

t 

g, 

h. 

Series  3 

b,  c,  d, 

e, 

f, 

g,  h 

Series  4 

b?c?d, 

e, 

f? 

<r9 

Series  5 

e?f?g. 

(5)  Devitrification  and  fine-mesh  crazing  are  developed 
in  this  group  at  cone  1  under  the  same  conditions  that 
were  noted  in  case  of  the  first  group.  The  essential 
difference  in  the  occurrence  of  these  effects  is  that  1st, 
fine-mesh  crazing  is  confined  to  glazes  having  a  lower 
equivalent  of  AL03  than  is  the  case  in  group  1 ;  2nd, 
devitrification  is  a  trifle  less  pronounced  in  this  group 
at  cone  1  than  it  was  in  Group  I  at  the  same  heat 
treatment. 

Fired  at  Cone  5. 

Series  1,  2,  3,  4,  (1)  Fine-mesh  crazing  occurs  to  the 
right  of  a  line  drawn  from  members  e  of  Series  1  to 
member  a  Series  5. 

(2)  Attack  on  body  by  the  glaze  is  evident  only  in 
the  glazes  which  are  crazed  in  fine  meshes. 

(3)  In  these  series  at  cone  5  as  in  every  other  in- 
stance so   far  noted,   members    e    and     f     (0.30-0.35 


42  FRITTED   GLAZES. 

A1203)  are  the  best  matured  and  freest  from  defects. 
(4)  Good  glazes,  including  only  those  which  are  free 
from  fine-mesh  crazing  and  are  well  matured,  are : 

Series  1     e,  f,  g,  h. 

Series  2     d,  e,  f,  g,  h. 

Series  3     d,  e,  f,  g,  h. 

Series  4     c,  d,  e,  f,  g,  h. 

Series  5     c,  d,  e,  f,  g,  h. 
Series  6,  7,  8.     (1)     Devitrification  is  shown  in  these  series 
to  a  less  extent  than  in  similar  series  of  Group  I  at 
the  same  temperature. 

(2)  Members  e  and  f  of  Series  6  and  7  are  fair 
glazes  but  contain  matter  in  suspension. 

(3)  Series  8  at  this  higher  oxygen  ratio  is  further 
matured  than  the  same  series  of  Group  I,  thus  em- 
phasizing the  fact  that  when  a  portion  of  a  raw  lead 
glaze  is  fritted,  the  oxygen  ratio  of  the  glaze  must  be 
more  than  1 :  2. 

Fired  at  Cone  10. 
The  appearance  of  the  glazes  of  Group  I  and  Group  II  at 
this  temperature  is  almost  identical.  The  statements 
made  concerning  Group  I  hold  true  of  Group  II  at 
cone  10,  except  perhaps  that  the  crazing  is  not  quite 
so  fine  meshed  in  Group  II. 

Conclusions  on  Group  II. 
The  good  glazes  developed  at  the  several  heats  are 
shown  in  the  following  table.  Those  glazes  having 
a  question  mark  beside  them,  it  is  believed,  would  have  de- 
veloped into  good  glazes  had  they  not  been  reduced  or 
blackened  by  deposition  of  carbon  in  firing. 


Series 

Cone  010 

Cone  06 

Cone  1 

Cone  6 

Cone  10 

1 

e?f? 

d  e  f  g  h? 

c  d  e  f  g  h 

d  e  f  g  h 

(a  to  h)? 

2 

e?f? 

d?  e  f  g? 

c  d  e  f  g  h 

c  d  e  f  g  h 

(a  to  h)? 

3 

e?f? 

d?  e  f  g? 

c  d  e  f  g  h 

c  d  e  f  g  h 

(a  to  g)? 

4 

c?  d  e  f  ?  g? 

c  d  e  f  g  h 

(a  to  f  )1 

5 

e?f? 

c  d  e  f  g  h 

overturned 

6 

c?  d?  e?  f  g  h? 

overburned 

7 

f  g?h? 

overturned 

8 

h? 

overturned 

FRITTED   GLAZES.  43 

2.  The  glazes  having  an  oxygen  ratio  of  1 :  2.5  show  a 
longer  heat  range  than  those  having  a  ratio  of  1 : 2.0. 

3.  Crazing  in  fine  meshes  is  decreasing  in  intensity 
with  increase  in  oxygen  ratio. 

4.  At  heat  treatments  below  cone  5,  fine-mesh  craz- 
ing is  greatest  with  lowest  A1203  content,  but  as  the  tem- 
perature increases  from  cone  5  to  cone  10,  this  crazing 
reaches  its  maximum  with  higher  and  higher  content  of 
A1203.  It  is  more  pronounced  where  B203  is  present  in  the 
largest  amounts.  Both  of  these  facts  agree  with  those  ob- 
served in  Group  I,  and  in  both  cases  the  conclusion  that 
fine-mesh  crazing  is  due  largely  to  the  ALO..  extracted  from 
the  body  and  diffused  throughout  the  glaze  magma  very 
slowly,  seems  to  be  fully  justified. 

5.  It  is  rather  difficult  on  first  thought  to  harmonize 
conclusions  3  and  4,  for  in  either  group,  when  considered 
separately,  increase  in  the  A1203  originally  added,  counter- 
acts a  tendency  to  craze  in  fine  meshes,  and  when  Groups 

I  and  II  are  compared  one  with  another,  it  is  seen  that 
increased  acidity  in  the  original  composition  of  the  glaze 
also  counteracts  fine-mesh  crazing  as  did  increased  A1203. 
Referring  to  the  description  of  the  fine-mesh  crazing  in 
Group  I  at  cones  5  and  10,  it  is  seen  that  members  a 
and  b,  which  exhibited  it  the  most  at  the  lower  heats,  were 
the  most  free  from  it  at  the  higher  heats.  Complete  satur- 
ation by  A1203  in  these  members  at  the  high  heats  and  in- 
complete diffusion  of  AL03  in  the  members  higher  in 
"original"  content  of  A1203  where  viscosity  is  the  greatest, 
is  offered  as  the  explanation  of  this  phenomenon.  In- 
creased acidity,  up  to  a  given  degree,  decreases  viscosity 
and  increases  ease  of  diffusion,  so  that  in  the  case  of  Group 

II  it  would  be  expected  that  the  A1203  taken  from  the  body 
would  be  more  readily  diffused  throughout  the  glaze 
magma,  thus  decreasing  the  liability  to  the  formation  in 
the  glaze  of  two  strata  of  different  composition. 

6.  From  the  fifth  conclusion  it  is  seen  that  in  this 


44  FRITTED  GLAZES. 

case,  increase  in  acid  does  not  increase  the  viscosity  of  the 
glaze,  but  on  the  contrary,  it  actually  makes  the  glaze 
less  viscous.  The  general  assertion,  therefore,  that  Si02 
added  to  a  fritted  glaze  makes  the  glaze  more  viscous  is 
not  true  at  these  lower  oxygen  ratios. 

7.  The  smoking  of  the  burns  at  cone  010,  05.  and  1 
suggested  a  cause  for  the  appearance  of  pinholes  in  glazes 
that  are  normally  free  from  them.  Mr.  Gray  at  the 
Boston  meeting  of  the  society  made  a  formal  inquiry  con- 
cerning this  phenomenon,  stating  that  it  occurs  very  freak- 
ishly, first  in  one  part  of  the  kiln,  then  in  another,  and  of- 
ten not  appearing  for  several  burns  at  at  time. 

Those  who  have  attempted  to  make  raw-lead  feldspar 
glazes  on  a  commercial  scale  have  learned  that  for  a  given 
temperature  the  glaze  cannot  vary  much  in  composition 
without  the  formation  of  these  pinholes  or  blisters,  as  they 
are  sometimes  called,  appearing  frequently  and  in  the  most 
unexpected  places. 

The  condition  of  the  glazes  of  Group  II  at  the  three 
lower  temperatures  suggests  that  if  carbon  is  deposited  on 
glazes  when  they  are  just  in  the  fritting  state  and  is  burned 
out  while  the  glaze  is  maturing,  the  carbonic  oxide  gas  gen- 
erated, not  only  forms  miniature  craters  when  it  escapes, 
but  also  reduces  the  lead,  and  perhaps  thereby  alters  the 
nature  of  the  compounds  formed  in  the  matrix,  making  the 
glaze  more  viscous  and  thus  preventing  the  healing  over  of 
the  craters  by  flowage  of  the  glaze. 

In  ordinary  firing,  the  temperature  is  raised  most  rap- 
idly just  before  and  during  the  time  that  the  glaze  ingredi- 
ents are  passing  from  the  sintering  through  the  fritting 
stages.  The  glazes  that  have  progressed  furthest  in  the 
glass-forming  stage  are  least  affected  by  carbon,  and 
hence  exhibit  fewer  pinholes  when  matured.  For  this  rea- 
son, Cornwall  stone  glazes  are  freer  from  pinholes  than 
feldspar  glazes. 

Group  III. 

A  table  of  the  compositions  of  the  64  glazes  composing 
this  group  follows  on  page  45. 


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46  FRITTED   GLAZES. 

Fired  at  Cone  010. 

This  group  was  badly  carbonized  in  this  heat  treat- 
ment, so  that  little  can  be  said  in  regard  to  it. 
Series  1,  2,  3.  From  the  fact  that  Series  1  of  Group  I  and 
series  1  of  Group  IV  developed  glasses  a  few  of  which 
were  fair  glazes,  it  is  safe  to  assume  that  similar  re- 
sults could  be  expected  from  the  same  series  of  this 
Group. 

Series  1  and  2  are  alike  in  that  the  glazes  are  grey- 
ish and  badly  boiled  and  honey-combed.     The  carbon 
has  either  delayed  the  glass  formation  or  has  not  per- 
mitted it  to  come  to  a  "quiet"  fusion.    While  the  lat- 
ter case  may  be  true  in  part,  it  is  believed  that  the  de- 
lay in  formation  of  a  glassy  structure  is  probably  the 
real  effect  of  the  presence  and  burning  out  of  carbon 
as  shoAvn  in  these  series. 
Series  4,  5,  6,  7,  8.     These  series  are  not  "boiled"  but  grade 
regularly  from  fused  masses  in    Series   4,    down    to 
porous  grey  coatings  in  Series  8. 
There  is  one  new  fact  shown  in  this  series  which  is 
not  borne  out  in  any  other  group  or  temperature,  but  which 
is  very  suggestive.    In  series  4,  5,  6,  7,  8,  the  glazes  lowest 
in  A1203,  i.  e.  glazes  a,  b,  and  c,  are  fused,  while  the  glazes 
having  0.30-40  Eqv.  A1203  are  porous.    In  series  1,  2  and  3 
at  this  heat  treatment  it  is  the  glazes  that  have  this  higher 
equivalent  of  Al2Oa  that  appear  to  be  the  most  matured. 
In  all  other  instances  noted,  these  same  glazes  would  be 
well  matured  when  those  lower  in  A1203  had  not  yet  fully 
formed  into  a  matured  glaze.     Can  it  be  then,  that  the 
glazes  lowest  in  A1203  at  this  oxygen  ratio  begin  fusion 
earliest  but  do  not  complete  their  maturity  as  early  as 
those  in  which  the  A1203  is  higher,  while  at  lower  oxygen 
ratio  it  is  the  glazes  higher  in  A1203  that  fuse  earliest? 


FRITTED  GLAZES.  47 

Fired  at  Cone  05. 

Series  1,  2,  3.  Members  a,  b,  c,  d,  e,  of  these  series  are 
well  matured  glazes. 

Member  f  in  each  case  appears  well  matured, 
but  is  pitted  in  a  way  that  gives  rise  to  a  suspicion 
that  these  series  had  been  slightly  smoked. 

Members  g  and  h  have  more  pronounced 
''smoke  defects"  than  f,  but  the  appearance  of  these 
members  does  not  warrant  a  statement  concerning 
their  maturity  at  this  heat. 

Series  4,  5,  6.  Are  all  very  much  alike  in  that  members 
a,  b,  and  c  are  devitrified  masses. 

Series  7,  8.  Are  stony  in  appearance  and  the  lower  mem- 
bers slightly  effloresced. 

Summary  of  Group  III  at  Cone  05. 

The  following  are  considered  as  being  "good"  at  this 
heat: 

Series  1    a,  b,c,  d,  e,f?g? 
Series  2     a,  b,  c,  d,  e,  f   g? 
Series  3    a,  b,  c,  d,  e. 

Fired  at  Cone  1. 

Series  1,  2,  3.  Members  a,  b,  c,  d,  e,  f  of  these  series  have 
fine  gloss,  and  are  in  the  main  free  from  crazing  and 
pinholing. 

Member  h  of  each  of  these  series  contains  sus- 
pended matter  that  resembles  a  flocculent  precipitate. 
Otherwise  member   h   gives  some  promise  as  a  glaze. 

Series  4.  Other  than  pin-holing  and  undissolved  material 
where  the  glazes  are  thickest,  members  a,  b,  c,  d,  e,  f 
and  possibly  g  are  well  fused  and  matured. 

Series  6,  7.  Members  a,  b,  and  c  are  just  passing  from  the 
devitrified  to  the  glassy  state.  The  remaining  mem- 
bers of  these  series  are  semi-glassy,  but  with  poor  evi- 
dence of  good  glass  possibilities. 


48  FRITTED  GLAZES. 

Series  8.     Members  a  and  b  are  devitrified,  while  the  re- 
maining members  show  a  fair  enamel  gloss. 

Summary  of  Group  III  at  Cone  1. 

The  following  glazes  are  considered  as  being  good  at 
this  heat : 

Series  1  a,  b,  c,  d,  e,  f,  g. 

Series  2  a,  b,  c,  d,  e,  f,  g? 

Series  3  a,  b,  c,  d,  e,  f,  g. 

Series  4  a,  b,  e,  d,  e,  f,  g? 

Series  5  e?f? 

Series  6  


Fired  at  Cone  5. 

Series  1,  2, 3,  4.  ( 1 )  All  members  of  these  series  are  well 
matured  glazes,  and  free  from  all  defects,  except  that 
of  crazing  in  the  lower  members. 

(2)  It  is  with  this  heat  treatment  that  glazes  of  this 
group  first  show  fine-mesh  crazing. 

Series  1,  members  a  and  b,  are  crazed  more  than  or- 
dinary, but  they  cannot  be  said  to  be  crazed  in  fine- 
mesh. 

Series  2,  members  a,  b,  and  series  3,  members  a,  b,  and 
c,  are  crazed  in  fine  meshes  where  the  glazes  are  thick, 
but  only  ordinarily  crazed  where  the  glazes  are  thin. 
In  series  4  again,  member  a  is  crazed  in  fine  meshes, 
while  b  and  c  are  only  moderately  crazed,  and  the  re- 
maining members  of  this  series  are  totally  free  from 
crazing. 

(3)  Nothing  can  be  ascertained  from  the  first  four 
series  regarding  the  relative  effect  of  Si02  and  B203 
on  the  coefficient  of  expansion  and  contraction  of  the 
glazes,  for  they  are  nearly  alike  in  this  respect,  with 
perhaps  slight  evidence  in  favor  of  B203  being  less  ef- 
fective than  Si02  in  the  reduction  of  the  coefficient. 
This  is  in  keeping  with  the  evidence  in  these  studies. 


FRITTED   GLAZES.  49 

Series  6.  This  series  is  described  by  itself  because  it  ex- 
hibits the  most  peculiar  phenomeu  of  haviug  little 
islands  of  very  minute  pin-holes  widely  scattered  over 
the  surface  of  an  otherwise  normally  matured  matrix. 
The  beginning  of  these  local  boiling  spots  was  noted 
in  the  cone  1  burn,  where  they  were  slightly  raised 
blisters.  At  cone  5,  however,  these  blisters  have  sub- 
sided, leaving  only  roughened  pin-holed  patches.  This 
phenomenon  is  unexplainable  at  present,  for  it  does 
not  resemble  in  any  way  the  pinholes  due  to  combus- 
tion of  entrapped  carbon. 

Other  than  the  defect  above  noted,  all  the  members  of 
this  series  were  well  matured. 

Series  7  and  8.  Members  a,  b,  c,  d,  e,  of  these  series  are 
devitrified  with  a  tendency  to  gloss  increasing  progres- 
sively from  a  to  g,  where  a  very  fair  gloss  is  shown. 

Summary  of  Group  III  at  Cone  5. 

(1)  All  members  of  the  first  series  of  this  group  may  be 
considered  as  good  glazes,  except  in  case  of  members 
a  and  b,  which  show  fine-mesh  crazing  to  some  extent. 

(2)  Fine-mesh  crazing  has  decreased  rapidly  with  in- 
crease of  the  oxygen  ratio  from  Group  II  to  Group 
III. 

(3)  B20,  is  less  effective  than  Si02  in  decreasing  coef- 
ficient of  expansion  and  contraction. 

(4)  Devitrification  is  exhibited  only  in  series  7  aud  8, 
and  in  these  this  phenomenon  decreases  as  the  AL03 
increases. 

(5)  Contrasting  the  appearance  of  devitrification  in 
Groups  I,  II,  III  it  is  noted  that  as  the  oxygen  ratio 
increases,  B203  becomes  more  effective  in  counteract- 
ing this  phenomenon  while  A1203  is  if  anything,  less 
so. 


50  FRITTED  GLAZES. 

Fired  at  Cone  10. 

1.  All  glazes  in  Group  III  are  decidedly  over-burned 
at  Cone  10,  with  the  possible  exception  of  members  a,  b,  c 
of  series  1  and  2,  and  members  b,  c,  of  series  3.  Thickly 
applied  as  they  are  in  these  studies,  members  of  series  1 
and  2  are  pinholed  where  thick  and  in  some  instances 
blistered  considerably;  but  near  the  edges  where  the  glazes 
have  run  thin,  neither  of  these  defects  is  noted.  The  writers 
hesitate,  therefore,  in  offering  an  opinion  in  regard  to 
members  of  series  1  and  2  at  this  temperature. 

2.  Fine-mesh  crazing  is  less  pronounced  in  this  group 
than  in  the  two  preceding,  and  there  is  also  less  difference 
in  the  extent  of  line-mesh  crazing  between  those  members 
which  were  originally  low  and  those  originally  high  in 
A1203. 

3.  In  this  Group  at  Cone  10,  boiling  of  the  mid- 
members  of  series  7  and  8  into  a  light  froth  has  reached  its 
maximum  intensity  for  the  entire  study.  This  would  seem 
to  indicate  that  with  the  oxygen  ratio  1:3  we  have  the 
greatest  fusibility. 

Summary  of  Group  III. 

(1)  Good  glazes  developed  in  this  group  at  the  vari- 
ous temperatures  are  as  follows : 

Series     Cone  010  Cone  05  Cone  1  Cone  6  Cone  10 

1  d?e?f?     abode  f  ?  g?      abcdefg      a?b?cdefgh      abcf 

2  d?e?f?      abcdef?g?      abcdefg      a?b?cdefgh      abc 

3  abcdef?  abcdefg      a?b?c?  defgh      abc 

4  abcdef         a?bcdefgh 

5  e?f?  a?bcdefgh 

6  a?bcdefgh 
7 

8 

(2)  The  points  of  superiority  of  the  glazes  of  this 
Group  over  those  of  Group  I  and  Group  II  are  as  follows: 

a.  Longer  heat  range. 

b.  Larger  range  of  composition  in  the  good  glazes. 


FRITTED   GL.AZES.  61 

c.  Less  crazing  of  the  fine-mesh  type  and  more  largely 
of  the  hair-line  type. 

d.  Less  tendency  to  become  dimmed,  scummed,  or  de- 
vitrified. 

e.  Brighter  gloss  under  all  heat  treatments. 

f.  Do  not  eat  into  the  body  to  a  noticeable  extent. 

(3)  At  the  oxygen  ratio  3: 1,  there  is  the  beginning 
of  an  acidity  range  in  which  the  largest  variation  in  glaze 
and  body  composition,  as  well  as  the  widest  range  in  heat 
treatment  is  possible. 

(4)  It  has  been  noted  that  the  glazes  of  lower  acid- 
ity attack  the  body  with  the  greater  avidity.  It  has  also 
been  noted  that  all  the  evidence  points  more  directly  to  the 
possibility  that  it  is  AL03  rather  than  Si02  which  the  glaze 
extracts  from  the  body,  for  without  regard  to  the  acidity 
of  the  glaze,  fine-mesh  crazing  ensued  only  when  the  orig- 
inal content  of  A1203  was  low.  Yet  at  this  higher  acidity, 
there  is  but  very  little  eating  into  the  body,  and,  conse- 
quently, fine-mesh  crazing  is  less  pronounced. 

It  is  hard  to  harmonize  the  two  apparently  opposing 
statements  last  made,  unless  we  grant  that  silica,  although 
an  acid,  does  not  have  a  solvent  power  proportional  to  the 
amount  present  in  any  given  case,  but  that  after  a  certain 
point  has  been  reached,  further  additions  of  silica  retards 
action  in  a  remarkable  way.  This  can  be  seen  in  Seger's 
Al203-Si02  curve.  Near  the  point  of  greatest  fusibility 
of  mixtures  of  various  silicate  materials,  there  seems  to  be 
quite  a  range  in  composition  without  much  variation  in 
point  of  fusion.  It  is  believed  that  in  the  case  of  the  fritted 
glazes  under  consideration,  the  point  of  greatest  fusibility 
is  not  far  from  that  of  which  oxygen  ratio  is  3 : 1,  and  that 
the  gradual  change  in  direction  of  the  curve  at  its  lowest 
point  is  sufficient  to  include  oxygen  ratios  ranging  from  3 
to  at  least  3.75  : 1.  Addition  of  acid  above  that  required  to 
establish  oxj'gen  ratios  within  these  ranges,  whether  it  is 
silica  or  boracic  acid,  seems  to  have  little  effect  upon  the 
glass,  as  it  goes  into  solution  very  slowly. 

P.  &  F— 4. 


52 


FRITTED  GLAZES. 


This  increased,  range  in  composition,  as  the  point  of 
maximum  fusibility  is  approached,  is  graphically  seen 
when  the  formulae  of  Seger  comes  5  to  27  inclusive  are  re- 
duced until  A1203  is  equal  to  unity.  By  this  reduction, 
the  Si02  is  uniformly  reduced  to  10,  so  that  if  plotted  on 
a  straight  line,  the  fractional  division  in  length  of  which 
represents  the  fractional  equivalent  of  RO  in  the  cone  for- 
mulae, it  will  appear  that  between  the  point  representing 
cone  6  the  distance  is  equal  to  that  covered  by  four  or  five 
of  the  highest  cones,  as  is  shown  in  the  following  figure. 


TPANS     AM.  CEO    SOC.    VOL    IX 


«         IS      21     a        17 


Punov  amo    rox        naufrel 


22  10      IS  16    <»        If 


CONE    NUMBERS 


III         II       1  ( 


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silicate  fusion. 

This,  then,  is  the  explanation  and  harmonization  of 
the  two  seemingly  opposite  facts  that:  fine-mesh  crazing 
is  due  to  the  extraction  of  A1203,  or  by  increasing  the 
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Group  IV. 


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54  FIUTTED    GLAZES. 

Group  IV  is  so  very  much  like  Group  III  in  every  way, 
that  it  need  not  be  described  in  detail. 

The  well  matured  glazes  of  this  group  are  as  follows : 

Series        Cone  010  Cone  05  Cone  1  Cone  6  Cone  10 

1  (btoe)?  abcde   abcdefg   abcdefgh?   abcdefg 

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3  abc     abedef    abcdefgh?    bede 

4  abedef    abedef  h? 

5  bed      abcdefgh 

6  abcdefgh 

7  c  d  e 


8 


Group   V. 


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55 


56  FRITTED  GLAZES. 

Group  V  shows  once  more  a  narrower  range  of  avail- 
able composition  and  limits  of  heat  treatment,  as  can  be 
judged  by  the  following  table  which  indicates  the  well 
matured  glazes. 

Series        Cone  010  Cone  05  Cone  1  Cone  6  Cone  10 

1  abed  abedefg  abedefgh    abed 

2  ab  abedef  abedefgh    abed 

3  ab  abedef  abedefgh    ab 

4  a  abedef  abedefgh 
6  abc  abedef 

6  a  bed 

7 

8 

In  Group  III,  at  Cone  10,  there  was  exhibited  some 
tendency  of  the  glazes  low  in  A1203  to  attack  the  body, 
while  in  Group  IV  but  a  slight  trace  of  such  action  was 
noted.  In  Group  V,  however,  eating  into  the  body  and 
a  consequent  development  of  fine-mesh  crazing  is  again 
seen. 

A  decided  opalescence  is  shown  in  member  a  of  series 
1,  2,  and  3  at  cone  10.  This  is  the  first  occurrence  of  this 
peculiar  super-saturation  effect. 

The  slow  rate  at  which  glass  formation  proceeds 
when  high  in  A1203,  is  very  strikingly  shown  in  the  case 
of  members  e,  f,  g,  h  of  series  1  and  2  of  Groups  IV  to  VII. 
At  010,  these  glazes  have  passed  thru  the  first  boiling  stage, 
and  are  fairly  smooth  glasses  containing  some  undissolved 
material.  It  requires  a  heat  treatment  extending  over  a 
range  of  10  cones  or  more  to  effect  complete  solution.  In 
some  cases,  notably  in  the  h  members,  the  glazes  will 
pass  into  the  second  boiling  stage  before  complete  solution 
of  all  the  incorporated  material  has  been  attained.  It  is 
noted  also  that  this  peculiarity  was  more  in  evidence  at  the 
lower  oxygen  ratios,  less  so  in  Group  III,  which  has  an 
oxygen  ratio  of  1:3,  and  then  again  becomes  progres- 
sively more  pronounced  as  the  oxygen  ratio  increases 
from  1 :  3  to  1 :  4.5. 


FRITTED  GLAZES.  57 

In  the  lower  oxygen  ratios,  glazes  having  a  medium 
A1203  content  had  the  longest  heat  and  composition  range. 
At  oxygen  ratio  of  1:3,  there  was  less  discrimination  in 
this  regard,  while  at  higher  oxygen  ratios,  it  is  the  glazes 
that  have  the  lowest  content  of  A1203  that  show  the 
longest  heat  and  composition  range. 

Explanation  of  this  reversal  in  effect  of  A1203  at  dif- 
ferent oxygen  ratios  will  be  given  after  describing  Groups 
VI  and  VII. 

Group  VI. 

The  table  on  page  58  shows  the  compositions  of  the 
sixty-four  glazes  composing  this  group. 


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FRITTED  GLAZES.  61* 

Fired  at  Cone  010. 

Group  VI  at  this  cone  is  wholly  immature.  No  efflor- 
escence. 

Members  of  Series  8  are  perfectly  flat  and  are  por- 
celain-like in  character. 

Fired  at  Cone  05. 

Series  1,  2,  3.  The  following  series  were  well  matured 
and  completely  in  solution. 

Series  1     a,  b.  c,  d. 
Series  2     a,  b,  c,  d. 
Series  3     a,  b,  c,  d. 
While  the  above  members  had  reached  good  maturity, 
members  g  and   h   of  the  same  series  were  just  sub- 
siding from  the  first  boiling  stage. 
Series  4,  5.    The  members  of  these  series  were  all  imma- 
ture glasses  more  or  less  full  of  undissolved  materials. 
Members  a  and  b  are  almost  clear. 
Series  6,  7,  8,  were  porcelanic  in  character,  decreasing  in 
glossiness    with    decrease    in    proportion    of    B203 
to  Si02. 

Fired  at  Cone  1. 

Series  1  and  2.  Members  a  to  g  inclusive  are  good  glazes. 
h  has  some  undissolved  material,  a  and  b  are  the 
onh-  members  of  this  series  that  are  crazed. 

Series  3  and  4.  Members  a  to  e  of  this  series  are  well 
developed,  g  and  h  have  not  settled  down  to  a  flat 
surface. 

Series  5  and  6.  Members  a,  b,  c,  d  of  this  series  are  prom- 
ising. The  remaining  members  have  not  reached 
quiet  fusion  as  yet. 

Series  7.  Member  a  is  fairly  good  glaze,  while  the  re- 
maining series  are  porcelanic  in  character. 

Series  8  are  porcelanic  in  appearance,  resembling  an  aver- 
age white  Bristol  glaze. 


60  FRITTED   GLAZES. 

Fired  at  Cone  5. 

Series  1  to  5  inclusive.  Every  member  of  these  series  ex- 
cept members  a  of  series  4  and  5,  are  good  glazes,  a 
of  series  4  and  5  are  crazed  in  extremely  fine  meshes. 

Series  6  and  7.  This  is  the  only  group  which  seems  to  have 
been  smoked  at  Cone  5,  and  as  a  consequence  there  is 
in  these  series  the  phenomena,  before  noted,  of 
patches  of  pin-holes  in  an  otherwise  smooth,  well-ma- 
tured matrix.  Members  c,  d,  e,  and  f,  present  such  an 
appearance  in  series  6  and  7. 

Series  8.  Member  d  of  Series  8,  Group  VI,  is  the  first 
instance  of  any  of  the  glazes  of  this  series  maturing, 
except  in  case  of    d    Series  8,  Group  1. 

Fired  at  Cone  10. 

The  only  good  glazes  in  this  group  at  Cone  10  were  Series 
5  and  6,  members  g  and  h.  All  other  glazes  were 
over-burned. 

Summary  of  Group  VI.    The  good  glazes  are  as  follows: 

Cone  10 


gb 

gh 


(2)     Opalescence  phenomena  are  shown  in  member 
"a"  of  Series  1,  2,  3,  4,  at  Cone  5  and  10. 

Group  VII. 

The  table  on  page  61  gives  the  composition  of  the  64 
glazes  composing  this  group. 


ries        Cone  010 

Cone  05 

Cone  1 

Cone  6 

1 

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3 

abed 

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a  to  h 

4 

a  b 

abode 

btoh 

5 

a  b 

abed 

btoh 

6 

abed 

c  d  e 

7 

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8 

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81 


$2  FRITTED   GLAZES. 


Fired  at  Cone  010. 


Group  VII  at  Cone  010  was  so  badly  carbonized  that  little 
can  be  said  in  regard  to  it  except  that  members  a  and 
b  of  the  first  three  series  were  approaching  the  stage 
of  quiet  fusion. 

Fired  at  Cone  05. 

Series  1,  2,  3.  Each  of  these  series  presents  a  well  ma- 
tured finish  at  the  low  A1203  end,  and  not  complete 
subsidence  from  the  first  boiling  stage  at  the  high 
A1203  end. 

In  the  first  four  members  of  Series  1,  and  the 
first  two  of  Series  2  and  3,  there  is  a  peculiar  milky 
appearance. 

Series  4  to  8.  These  series  show  successive  gradation  from 
glass  to  a  porcelanic  mass. 

The  members  of  Series  8,  and  especially  those  low 

in  A1203,  resemble  very  much  the  opalite  tile  bodies. 

Fired  at  Cone  1. 

Series  1,  2,  3.  All  members  of  these  series  have  reached 
the  first  quiet  fusion  stage. 

The  following  glazes  appear  to  have  reached  their  full 
maturity : 

Series  1    a,  b,  c,  d,  e  f. 
Series  2    a,  b,  c,  d,  e,  f. 
Series  3     a,  b,  c,  d. 
In  members  b,  c,  d,  e  of  Series  1,  and  e,  d,  of 
Series  2  and  3,  the  milky  cloudiness  noted  at  Cone  05 
has  developed   into   opalescent   crystals.     This  phe- 
nomenon reaches  its  maximum  development  in  mem- 
ber c  of  Series  1. 
Series  4,  5.     These  series  have  not  progressed  much  be- 
yond the  first  boiling  stage  at  the  high  A1203  end,  but 
are  well  matured  at  the  low  Al-O.  end. 


KR1TTKD   GLAZES.  68 

The    following   are    the   members    developed    at 
Cone  1 : 

Series  4     a,  b,  c. 
Series  5     a,  b,  e. 
Series  6,  7,  8.     Are  all  immature. 

Fired  at  Cone  5. 

The  following  glazes  of  this  group  were  well  ma- 
tured at  Cone  5 : 

Series  1     a,  b,  c,  d,  e,  f,  g,  h. 
Series  2    a,  b,  c,  d,  e,  f,  g,  h. 
Series  3    a,  b,  c,  d,  e,  f,  g,  h. 
Series  4     a,  b,  e,  d. 
Series  5     a,  b. 
Series  G     a. 
Opalescence    was    developed    in    the    following 
glazes : 

Series  1     a,  b,  c,  d,  e. 
Series  2     a,  b,  c. 
Series  3     a,  b. 
Series  4     a. 
The  opalescent  effects  in    c,  Series  1,  are  the  most 
perfect  the  writers  have  ever  seen. 

Remaining  members  of  the  group  are  either  over- 
fired  or  immature. 

Fired  at  Cone  10. 

Summary:  Since  there  are  only  five  distinctive  fea- 
tures brought  out  in  the  Group  at  Cone  10,  detailed  des- 
cription of  the  separate  series  will  not  be  given. 

(1)  Members  a  of  Series  1  and  2  were  the  only 
glazes  of  this  group  that  were  developed  at  Cone  10.  The 
others  were  either  immature  or  over-burned. 

(2)  Opalescent  effects  at  this  temperature  have 
passed  from  the  crystalline  to  flowed  opaque  streaks. 


64  FBITTED   GLAZES. 

(3)  Quick  passage  from  an  immature  to  an  over- 
burned  condition  of  these  glazes  high  in  A1203  is  as  pro- 
nounced in  this  group  as  in  Group  V  and  VI. 

(4)  This  narrowness  in  maturing  range  is  more 
pronounced  in  the  series  low  in  B203. 

(5)  Attack  on  the  body  by  the  glazes  is  not  notice- 
able even  at  Cone  10. 

SUMMARY  OP  RESULTS. 

Heat  Range. 

It  was  thought  that  a  study  of  the  heat  range  of  the 
several  glazes  could  best  be  made  by  taking  them  up  in 
eight  charts,  on  the  basis  of  an  equal  A1203  content.  Ac- 
cordingly curves  have  been  drawn  in  which  the  Si02-B203 
ratio  is  plotted  on  the  abscissa,  and  the  oxygen  ratio  on  the 
ordinate.  The  curves  thus  plotted  show  the  thermal  boun- 
daries within  which  the  glazes  will  mature.  In  drawing 
these  curves  accidently  good  or  bad  glazes,  lying  wholly 
outside  of  their  proper  areas  were  ignored.  The  thermal 
limits  shown  are  the  extremes.  It  is  obvious  that  it  would 
not  be  advisable  to  reach  these  extremes  in  practice. 

Figure  3. 

Figure  3  shows  the  thermal  boundary  limits  of  maturity 
for  the  a  glazes,  which  have  an  Al2Oa  content  of  0.10 
equivalents. 

The  areas  included  within  these  lines  have  been 
designated  by  Roman  numerals,  in  order  to  make  their 
identification  in  the  text  easier. 

The  glazes  to  the  right  of  the  thermal  lines  are  well 
matured,  within  the  limits  shown. 

Glazes  plotted  in  the  area  marked  O  do  not  reach 
their  full  maturity  before  Cone  1  is  attained,  and  are  on 
the  verge  of  being  over-burned  at  Cone  5. 

Glazes  plotted  in  the  upper  portion  of  area  I  have  a 


FRITTBD   GLAZES. 


66 


S^OgVNON     dflOBO 


66  FRITTED  GLAZES. 

heat  range  from  Cone  05  to  5,  while  those  in  the  lower 
portion  of  this  area  have  a  range  only  from  Cone  05  to 
Cone  1. 

Glazes  plotted  in  area  II  have  a  heat  range  from  Cone 
05  to  5.  As  the  B203  increases  in  proportion  to  Si02,  the 
glazes  of  this  area  have  their  heat  range  increased,  until 
those  highest  in  B203  can  withstand  Cone  10.  The  glazes 
having  a  heat  range  that  includes  Cones  05  and  10  are 
those  plotted  in  areas  III  and  VI. 

Glazes  plotted  in  area  IV  are  either  immature  or 
over-burned.  Those  having  an  oxygen  ratio  of  4.0  to  4.5 
are  over-burned  at  Cone  5,  and  the  others  not  until  Cone  10 
has  been  reached. 

Glazes  in  area  V  have  a  heat  range  from  Cone  1  to 
Cone  5. 

Glazes  in  area  VII  practically  have  no  heat  range. 
Below  Cone  10  they  are  so  thoroughly  crazed  in  fine 
meshes  that  they  must  be  ruled  out  as  glazes,  although 
where  B203  is  high,  they  may  be  good  glasses.  At  Cone 
10  they  are  well  developed  glazes,  being  crazed  only  in 
hair  lines. 

The  glazes  containing  only  0.10  equivalents  A1203  have 
the  longest  heat  range  with  the  O.  R.  range  3.0  to  4.0, 
and  Si02-B203  ratio  range  of  0.25  to  0.17. 

Figure  4. 

Glazes  plotted  in  the  upper  portion  of  area  I  are  over- 
burned  at  Cone  5,  while  those  plotted  in  the  lower  portion 
or  area  I  and  near  to  the  Cone  10  thermal  curve  are  not 
over-burned  until  Cone  10  is  reached,  while  those  lying 
further  away  from  the  Cone  10  curve  remain  immature 
until  Cone  10  has  been  reached. 

Glazes  plotted  in  area  II  have  a  heat  range  from 
Cone  05  to  Cone  5. 

Glazes  plotted  in  area  III  close  to  the  Cone  05  ther- 
mal line  are  good  at  Cone  05  and,  as  the  content  of  B203 


FRITTED   GL.AZB8. 


P7 


saifliAiow  dnouy 


SCM-LVU 


N39AXO 


P,  A  F.-6. 


68  FRITTED  GLAZES. 

increases  at  the  expense  of  Si02,  the  heat  range  increases 
until  those  plotted  in  area  IV  have  a  range  from  Cone  05 
to  Cone  10. 

The  one  glaze  plotted  in  area  V  has  a  heat  range  from 
Cone  010  to  Cone  10. 

Glazes  in  area  VI,  like  those  plotted  in  a  correspond- 
ing area  in  Figure  3,  are  in  some  cases  good  glasses  at 
heats  lower  than  Cone  10,  but,  owing  to  their  extremely 
fine  mesh  crazing,  they  cannot  be  classed  as  glazes  until 
Cone  10  is  reached. 

Aside  from  glaze  b  of  Series  1,  Group  IV,  the  glazes 
having  the  longest  heat  range,  with  A1203  content  equal 
to  0.15  equivalent,  are  found  in  Series  1,  2  and  3  of  Group 
III  to  VI,  inclusive.  These  glazes  have  a  heat  range  from 
Cone  05  to  Cone  10,  inclusive. 

Figure  5. 

Glazes  plotted  in  area  I  are  immature  until  Cone  10 
is  reached,  and  then  they  pass  directly  into  an  over-burned 
condition. 

Glazes  plotted  in  the  upper  portion  of  area  III  have  a 
heat  range  from  Cone  1  to  Cone  5 ;  those  contiguous  to  the 
Cone  1  thermal  curve  are  barely  matured  at  Cone  1,  and 
those  nearer  the  thermal  curve  of  Cone  5  are  decidedly  im- 
mature at  Cone  1. 

Glazes  plotted  in  the  upper  portion  of  area  III  have  a 
heat  range  that  extends  from  Cone  05  to  5,  while  the  heat 
range  of  those  in  the  lower  portion  of  area  III  extends 
close  to  Cone  10. 

Glazes  in  area  IV  have  a  heat  range  from  Cone  05  to 
Cone  5.  Those  plotted  near  the  Cone  05  thermal  curve  are 
just  matured  at  Cone  05. 

Glazes  plotted  in  area  VII  have  a  heat  range  from 
Cone  5  to  Cone  10,  being  not  quite  matured  at  Cone  5. 

Glazes  plotted  in  area  VIII  have  a  heat  range  from 
Cone  1  to  Cone  10,  being  barely  matured  at  Cone  1. 


FRITTED   GLAZES. 


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S-OIXVy        N19AXO 


TO  FRITTED   GLAZES. 

Glazes  plotted  in  area  IX  have  a  heat  range  from 
Cone  1  to  Cone  10,  being  fully  matured  at  Cone  1. 

Glazes  plotted  in  area  V  have  a  heat  range  from 
Cone  05  to  Cone  10,  being  fully  matured  at  Cone  05. 

The  maximum  heat  range  then  of  glazes  containing 
0.2  equivalent  of  A1203,  is  to  be  found  in  Series  1,  2  and  3 
of  Groups  III  to  V,  inclusive. 

Figure  6. 

Glazes  plotted  in  area  I  are  over  burned  at  Cone  5. 

Glazes  plotted  in  area  II  are  immature  at  Cone  5,  but 
over-burned  at  Cone  10. 

Glazes  plotted  in  area  III  have  a  heat  range  from 
Cone  1  to  Cone  5.  Those  of  the  lower  portion  are  harder 
than  those  of  the  upper  portion  of  this  area. 

Glazes  plotted  in  area  IV  have  a  variable  heat  range, 
those  having  an  oxygen  ratio  of  4.5  showing  a  range  from 
Cone  1  to  Cone  5;  those  having  an  oxygen  ratio  of  3.5  to 
4,  inclusive,  showing  a  range  from  Cone  1  to  Cone  10,  when 
B203  is  high,  while  those  having  an  oxygen  ratio  of  2.5  to 
3.5  show  a  range  from  Cone  05  to  Cone  5.  No  explanation 
of  this  variableness  in  heat  range  of  the  glazes  of  the  area 
can  be  offered. 

Glazes  plotted  in  area  V  have  a  heat  range  of  Cone 
1  to  10. 

Glazes  plotted  in  area  VI  have  a  heat  range  from  Cone 
05  to  Cone  10. 

Glazes  plotted  in  area  VII  have  a  heat  range  from 
Cone  010  to  Cone  10. 

Glazes  plotted  in  area  VIII  have  a  heat  range  from 
Cone  010  to  Cone  5. 

Glazes  plotted  in  area  IX  have  a  heat  range  that  aver- 
ages from  Cone  05  to  Cone  5. 

Glazes  plotted  in  area  X  have  a  heat  range  that  ex- 
tends onlv  from  1  to  10. 


FRITTED   GLAZES. 


71 


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72  FRITTED  GLAZES. 

Glazes  plotted  in  areas  XI  and  XII  have  a  heat  range 
from  05  to  10,  those  in  area  XI  being  immature  and  those 
in  area  XII  being  fully  matured  at  Cone  05. 

The  glazes  having  the  longest  heat  range  are  those 
plotted  in  areas  V,  VI,  VII,  VIII,  IX  and  XII,  or  those 
having  an  oxygen  ratio  of  from  2.5  to  4.0,  inclusive,  and  a 
Si02-B2Os  ratio  of  0.17  to  0.25. 


Figure  7. 

The  description  of  Figure  6  applies  to  Figure  7,  ex- 
cept for  minor  details. 


FRITTED   GLAZES. 


73 


cu3flwnN  aiyotiO 


Cpl-LVU 


KJ30AX0 


74  FRITTED   GLAZES. 


Figure  8. 

Glazes  of  all  areas  except  I,  VII  and  VIII  have  heat 
ranges,  the  maximum  points  of  which  are  Cone  5  in  the 
case  of  II  and  III,  and  Cone  10  in  the  case  of  IV,  V  and 
VI,  and  the  minimum  points  of  which  lie  in  the  curve 
bounding  them  on  the  right,  except  in  case  of  III. 

The  longest  heat  range  shown  in  the  case  of  glazes 
having  0.35  Eqv.  content  of  A1203,  is  found  at  the  oxygen 
ratios  of  2.0  to  3.50,  inclusive,  and  a  Si02-B203  ratio  of 
from  0.25  to  0.17. 


KBITTBD  OLAZK8. 


75 


Sa39wnN 


dnouO 


.JOLLVii 


N3  9AXO 


76  FBITTBD   GLAZES. 


Figure  9. 

Glazes  plotted  in  area  I  were  immature  even  at  Cone 
10,  except  when  the  oxygen  ratio  is  low. 

Glazes  plotted  in  area  II  have  a  heat  range  from 
Cone  1  to  Cone  5. 

Glazes  plotted  in  area  III  have  a  heat  range  from 
Cone  1  to  Cone  10. 

Glazes  plotted  in  area  IV  have  a  heat  range  from 
Cone  05  to  5  or  more,  being  somewhat  immature  at 
Cone  05. 

Glazes  plotted  in  area  V  have  a  heat  range  from  Cone 
05  to  nearly  Cone  10,  being  fully  matured  at  Cone  05. 

Glazes  plotted  in  area  VI  have  a  heat  range  that  ex- 
tends from  a  little  above  Cone  05  to  Cone  10,  while  those  of 
area  VII  have  a  heat  range  that  extends  from  Cone  05 
to  Cone  10. 

The  glazes  plotted  in  area  VIII  have  a  heat  range 
from  Cone  010  to  10,  inclusive. 

The  glazes  having  an  A1203  content  of  0.40  equivalents 
and  showing  the  longest  heat  range,  are  found  within  the 
oxygen  ratios  of  2.0  to  3.5,  inclusive  and  Si02-B203  ratio 
from  0.25  to  0.20,  inclusive. 


FRITTED   GLAZES. 


77 


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78  FRITTED  GLAZES. 


Figure  10. 

Glazes  plotted  in  area  I  were  not  matured  at  Cone 
5  or  10. 

Glazes  plotted  in  area  II  were  good  at  Cone  5.  Those 
to  the  extreme  left  were  barely  matured  and  had  a  short 
heat  range,  while  those  to  the  extreme  right  were  fully 
matured  and  had  a  heat  range  from  Cone  5  almost  to  Cone 
10  in  all  cases  and  in  some  cases  from  Cone  1  to  10. 

Glazes  plotted  in  area  III  have  a  heat  range  from  Cone 
1  to  almost  10,  being  over-fired  at  Cone  10. 

Glazes  plotted  in  area  IV  have  a  heat  range  that 
varies  from  Cone  05  to  Cone  10  and  those  in  area  V  from 
Cone  010  to  Cone  10,  being  rather  immature  at  Cone  010. 

Glazes  plotted  in  area  VI  were  good  from  Cone  010  to 
Cone  10,  inclusive. 

Glazes  plotted  in  area  VII  have  a  heat  range  from 
Cone  1  to  Cone  10. 

The  greatest  heat  range  shown  by  glazes  containing 
0.45  Al2Oa  is  found  within  the  oxygen  ratio  of  2.0  to  3.0, 
and  a  Si02-B203  ratio  that  averaged  about  as  1 :  2.0. 


FRITTED   OLAZKS. 


79 


Sd3fllNAN 


COIJLVM        N30AX0 


80 


FRITTED  GLAZHS. 


Summary  on  Heat  Range:  (1)  In  the  following 
table  is  shown  the  limits  of  composition,  within  which  the 
longest  heat  range  was  obtained. 


Eqv.  A1303 

Oxygen 

SKX-B,  Oa 

Average  Maximum  Heat  Range 

Content 

Ratio 

Ratio 

in  Cones 

0.10 

3.0-4.0 

0.25-0  17 

05-10 

015 

3.0-4.0 

0.25-0.17 

05-10 

0.20 

3.0-3.75 

0.25-0.17 

05-10 

0.26 

2.5-3  75 

0.25-0.17 

05-10 

0.30 

2.0-3.50 

0.25-0.17 

010-10 

0.35 

2.0-3.50 

0.25-0.17 

010-10 

0.40 

2.U-3.50 

0.25-0.20 

H5-10 

0.45 

2.0-3  50 

0.25-0.20 

05-10 

Composition  Range. 

Member  a.    Al2Os=0.10  Eqv. 

The  following  glazes  are  well  matured  and  crazed 
only  in  hair  lines  at  the  cones  indicated : 

Free  From  Crating 


Groups 

Series 

Heat  Range  In  Cones 

I 
II 

None 
None 

m 

J  1,  2,  3, 

14,5, 

05-5 

1 

IV 

;i,2 

is 

05-10  in  elusive 
06-5  inclusive 
5 

V 

/ 1,  2,  3, 
1 4,  5,6, 

06-5  inclusive 
1 

VI 

fl,2,  3, 
]4,  5, 
(6,7, 

05-5  inclusive 
06-5  inclusive 
1 

vn 

fl,2,8, 
14,5,6,7, 

05-5  inclusive 
1 

The  others  are  matured  at  heat  ranges  shown  in 
Figure  3,  but  are  crazed  in  fine  meshes. 

Member    b.     Al2O3=0.15  Eqv. 

The  following  glazes  are  well  matured,  and  crazed,  if 
at  all,  only  in  hair  lines  at  cones  indicated.  Those  free 
from  crazing  are  indicated  in  the  fourth  column. 


FRITTED   GLAZES. 


81 


Groups 
I 
II 

Series 

None 
None 

Heat  Range  Id  Cones 

III 

i  1,2,  3, 

I  4,  5,  6, 

05-5 

1 

IV 

/I,  2,  3, 

06-5  inclusive 
1 

V 

|  1,  2,  3, 

05-6  inclusive 
1 

VI 

/I,  2,  3, 
14,5,6, 

05-5  inclusive 
1-6  inclusive 

VII 

/  1,  2,  3, 
14,5, 

05-6  inclusive 
1 

Free  From  Crating 


at  cone  1 

at  cone  1 

at  cone  1 

at  cone  1 

Series  1  at  cone  6 

Series  2  3  4  5  6  at  Cone  1 

Series  1  at  Cone  05-5 

Series2,  3,  4,  6,  at  Cone  1 


The  remainder  are  matured  at  heat  ranges  shown  in 
Figure  4,  but  are  crazed  in  fine  meshes. 

Member    c.     AJ2O8=0.20  Eqv. 

The  following  glazes  are  well  matured  and  crazed,  if 
at  all,  only  in  hair  lines  at  cones  indicated.  Those  free 
from  crazing  are  indicated  in  the  fourth  column. 


Group 

Series 

I 
II 

2,3 

III 

J  1,  2,  3, 
14,5,6, 

IV 

U,2, 
18,4, 

V 

H,2,3, 
14,5, 

VI 

11,2,3, 
14,5, 

VI 

J  1,8, 

1 3,  4,  5, 

Heat  Range  In  Cones 


1 

05-6 

1 

05-6  inclusive 

1-5  inclusive 

05-6  inclusive 

1-6 

05-6  inclusive 

1-5  inclusive 

05-5  inclusive 

1-5  inclusive 


Free  From  Crazing 


at  cone  1 

at  cone  1 
at  cone  1-5  inclusive 
at  cone  1-5  inclusive 
cone  1-5  inclusive 
cone  1 

cone  1-5  inclusive 
cone  1 


The  remainder  are  matured  at  heat  ranges  shown  in 
Figure  5,  but  are  crazed  in  fine  meshes. 

Member    d.     Al2OR=0.25  Eqv. 

The  following  glazes  are  well  matured  and  crazed,  if 
at  all,  only  in  hair  lines  at  cones  indicated.  Those  free 
from  crazing  are  indicated  in  the  fourth  column. 


82 


FRITTED  GLAZES. 


Group 

I 

II 

Series 

Heat  Range  in  Cones 

Free  From  Crazing 

5  2,  8, 

( 4,  5,  6, 

010-1  inclusive 
06-5  inclusive 
5 

Series  4  at  Cone  6 

in 

(1,2,3, 
(5,6 

05-5  inclusive 
1-5  inclusive 
5 

Series  8, 4  at  cones  1-5 

IV 

(1,2, 

05-5  inclusive 
1-6  inclusive 
5 

1,  2,  3,  at  cone  1 
1,  2,  3,  at  cones  1-6 

V 

12;  3, 

05-010  inclusive 
1-5  inclusive 

1,  2,  3,  at  cones  1-5 

VI 

(1,2,3,4, 
1  5,  6,  7, 

1-5 
5 

2,  3,  4,  at  cones  1-5 
5,  6,  7,  at  cone  5 

VII 

i  1,  2,  3, 

1-5 
5 

1,  2,  3,  at  cones  1-6 
4  at  cone  5 

The  remaining  glazes  are   matured   at   heat   ranges 
shown  in  Figure  6,  but  are  crazed  in  fine  meshes. 


Member   e. 

AZ2O3=0.30  Eqi 

Group 

Series 

Heat  Range  in  Cones 

I 

{i,M, 

05-5  inclusive 
1-5  inclusive 

II 

\  2,  3, 
(  4,  5,  6, 

06-6  inclusive 
1-6  inclusive 
5 

III 

11,2,8, 

]4,5, 

(6, 

05-6  inclusive 
1-6  inclusive 
5 

IV 

fl,2,3, 
I  6-8 

1-10  inclusive 

1-5 

6 

V 

1  1,  2,  3,  4, 
15,6, 

1-5  inclusive 
5 

VI 

J  1,  2,  3,  4, 

1-6  inclusive 
5 

VII 

J  1.1, 

|8,  4, 

1-5  inclusive 
5 

Free  from  From  Crazing 

None 


4,  5,  6,  at  cone  5 

1,2,3,4,  at  cones  1-5 

1,2,3,  at  cones  1-10 
4  at  cones  1-5 

5,  6,  at  cone  6 

1,2,  3,4,  at  cones  1-5 

6,  6,  at  cone  6 

1,2,3,4,  at  cones  1-5 

1,  2,  at  cones  1-5 
3,  4,  at  cone  5 


The  remaining  glazes  are  matured   at    heat    ranges 
shown  in  Figure  7,  but  are  crazed  in  fine  meshes. 


FRITTED   QIiAZBfl  87 

as  to  produce  a  cloudy  or  milky  appearance.  When  over- 
tired, the  effect  is  similar  to  the  curdiness  produced  in  a 
glaze  by  addition  of  raw  tin  oxide.  When  fired  to  the  best 
maturity  of  the  glaze,  the  opalescent  effects  produced  by 
supersaturation  with  Si02  are  distinctly  crystalline. 

i  5 1  While  the  writers  have  advanced  the  opinion 
that  this  opalescent  effect  is  produced  by  supersaturation 
and  separating  out  of  Si02,  they  can  offer  no  evidence  con- 
tradictory to  the  hypothesis  that  the  material  thus  separ- 
ated out  is  a  borate.  High  content  of  B203  is  required,  and 
the  opalescence  here  described  not  only  does  not  appear 
unless  B2Os  is  proportionately  high,  but  it  increases  in  in- 
tensity and  is  produced  in  wider  and  wider  variation  as 
the  B203  increases  in  proportion  to  Si02.  The  writers  have 
based  their  opinion  that  this  phenomenon  is  due  to  the 
supersaturation  of  the  glaze  by  Si02,  on  the  chemical  com- 
position of  milky  opals  as  they  occur  in  nature. 

ATTACK  ON  THE  BODY. 

The  supposition  has  been  advanced  that  A1203  is  the 
principal  constituent  taken  from  the  body  by  the  glaze. 
The  facts  that  support  this  assumption  are  as  follows : 

(1)  The  glazes  lowest  in  A1203  (0.10—0.20  Eqv.) 
show  this  phenomenon  to  the  greatest  extent. 

(2)  The  glazes  lowest  in  Al20:i  show  the  fine-mesh 
crazing  which  results  from  incorporating  constituents 
from  the  body  into  the  glaze  layer  that  is  contiguous  to  the 
body,  irrespective  of  oxygen  ratio.  It  is  a  fact,  however, 
that  fine-mesh  crazing  decreases  as  the  oxygen  ratio  in- 
creases, but  this  is  attributed  to  the  approximate  satura- 
tion of  the  matrix  with  acid,  as  is  evidenced  in  the  appear 
ance  of  opalescence  as  early  as  Group  VI. 

(3)  Glazes  having  a  low  oxygen  ratio,  and  low  A120,t 
content,  decrease  in  fine-mesh  crazing  as  the  intensity  of 
heat  increases. 

(4)  Glazes  that  craze  in  fine  meshes  when  thick  are 
crazed  only  in  hair  lines  when  thin. 

P.  A  F.— 7. 


88  FRITTED   GLAZE?. 

(5)  Increase  of  originally  added  A1203  decreases 
fine-mesh  crazing  under  less  intense  temperatures,  but  in- 
creases it  as  the  temperature  becomes  more  intense. 

(6)  Fine-mesh  crazing  is  certainly  due  to  lack  of 
homogeniety  in  the  composition  of  the  upper  and  lower  sur- 
face of  the.  glaze  layer,  and  this  lack  of  homogeniety  is 
more  pronounced,  the  more  viscous  the  glaze  is  rendered 
by  the  incorporated  constituent. 

(7)  Fine-mesh  crazing  developed  at  temperatures 
below  Cone  5,  can  be  counteracted  by  increasing  the  Al2Os 
content  of  the  glaze.  If  it  is  developed  at  higher  heats,  it 
can  be  counteracted  by  decreasing  the  A1203  content,  es- 
pecially when  the  oxygen  ratio  is  low. 

PIN-HOLING. 

Pin-holing  and  blistering  has  been  proven  in  these 
studies  to  be  primarily  due  to  the  combustion  of  carbon 
that  had  been  entrapped  during  the  fritting  stage  of  glaze 
formation.  The  glazes  that  come  to  a  quiet  fusion  earliest 
are  less  likely  to  exhibit  this  defect.  Raw  feldspar  glazes 
show  this  defect  more  than  the  raw  Cornwall  stone  glazes, 
but  as  was  shown  by  Mr.  Coulter1  the  stone  glazes  are 
more  fusible,  and  have  a  longer  heat  range,  and  therefore 
are  subjected  for  a  shorter  time  to  the  influence  of  carbon 
in  the  fire  gases  before  quiet  fusion  sets  in.  As  the  evidence 
in  this  study  in  the  case  of  fritted  glazes  agrees  with  simi- 
lar x>henomena  observed  in  raw  lead  glazes,  there  seems 
to  be  no  doubt  that  the  statements  in  regard  to  the  cause 
of  pin-holing  given  above  are  correct. 

In  conclusion  the  writers  wish  to  express  their  appre- 
ciation of  the  hearty  support  and  substantial  encourage- 
ment they  received  from  the  authorities  of  the  University 
of  Illinois  in  general,  and  from  Professor  C.  W.  Rolfe,  Di- 
rector of  the  Ceramic  Department,  in  particular.  Had 
they  not  granted  every  facility  within  their  power,  the  exe- 


'Trans.  Am.  Cer.  Soc.  Vol .  VII,  p.  356. 


K  KITTED   GLAZES.  81* 

cution  of  this  study  In  its  details  would  have  been  impos- 
sible. We  wish  also  to  express  our  further  appreciation  of 
the  privilege  of  offering  the  results  of  this  research  to  the 
American  Ceramic  Society  previous  to  its  issuance  as  a 
University  Bulletin. 

DISCUSSION. 

The  Chair:  By  the  presentation  of  this  very  able 
paper,  I  am  impressed  at  the  outset  with  the  growing  im- 
portance of  making  provision,  as  soon  as  our  financial  con- 
dition shall  warrant  it,  for  the  publication  of  such  papers 
prior  to  our  meetings,  so  that  we  may  study  them,  and  be 
better  able  to  discuss  them  when  we  come  to  the  meeting. 
We  have  realized  this  necessity  during  past  sessions,  but 
the  need  is  growing  more  apparent  all  the  time.  Still  this 
paper  contains  so  many  interesting  points  that  we  ought 
to  have  a  general  discussion  on  it. 

Mr.  Par  melee:  I  feel,  of  course,  wholly  unable  to 
discuss  the  detail  of  this  work.  It  certainly  is  a  big  sub- 
ject, and  I  think  brings  pleasure  to  all  members  to  see  such 
a  substantial  contribution  on  it.  Did  I  understand  you  to 
say,  Mr.  Fox,  that  coke  was  used  as  a  fuel  ? 

Mr.  Fox:     Yes,  sir. 

Mr.  Parmelee:  Then  how  do  you  attribute  the  pin- 
holing  to  carbon  deposited  on  the  glaze?  In  what  form  is 
it  deposited  on  the  glaze? 

Mr.  Purdy:  Coke  certainly  contains  carbon,  which 
on  combustion  is  driven  off.  Not  all  the  carbon,  however, 
is  oxydised  to  either  carbon  monoxide  or  carbon  dioxide; 
some  of  it  may  be  carried  by  the  draft  as  particles  of  free 
carbon. 

Mr.  Parmelee:  Then  these  are  particles  of  fuel,  car- 
ried mechanically  and  deposited  mechanically? 

Mr.  Purdy:  We  know  that  black  smoke  contains 
about  two  percent,  of  carbon,  and  when  the  coke  is  freshly 
thrown  on  the  grate  bars  there  will  be  an  evolution  of 
smoke  or  black  gases,  and  that  is  sufficient  to  cause  a  de- 


90  FRITTED   GLAZES. 

posit  of  carbon  on  the  glaze,  notwithstanding  the  fact  that 
the  saggers  are  thoroughly  sealed. 

The  Chair :  You  assume  that  in  all  of  the  kiln  firings 
you  formed  this  deposit,  but  in  some  were  able  to  free  the 
ware  from  it  more  completely  than  in  others? 

Mr.  Purdy:  No.  When  deposited  prior  to  fusion, 
then  the  carbon  would  be  incorporated  and  cause  pin- 
holing  ;  but  when  deposited  after  the  quiet  stages  of  fusion 
has  been  reached,  the  glaze  would  not  then  be  affected. 

The  Chair :  But  all  appear  to  have  the  deposit  in  the 
earlier  stage,  but  some  are  able  to  free  themselves? 

Mr.  Purdy :  No.  Carbon  is  effective  in  producing  pin- 
holes only  when  it  becomes  incorporated  into  the  glaze.  If 
carbon  is  deposited  on  the  glaze  during  the  boiling  period, 
it  will  become  incorporated.  A  glaze  which  has  its  initial 
boiling  period  early,  or  which  has  a  very  short  or  mild 
boiling  period,  will  not  be  affected  as  much  as  the  glaze 
that  boils  later  and  consequently  harder. 

The  Chair:  I  will  ask  Mr.  Purdy  what  method  he 
used  to  determine  over-firing? 

Mr.  Purdy.  We  determined  that  by  its  appearance, 
i.  e.  when  it  began  to  pass  into  the  second  boiling  stage.  I 
do  not  know  why  it  should  boil  the  second  time,  but  it  does. 

Mr.  Par  melee:  I  have  never  had  any  experience  in 
coke  firing,  but  am  surprised  to  learn  that  in  burning  coke 
you  have  black  smoke. 

Mr.  Purdy :  You  will  have  some ;  and  if  you  put  one 
speck  of  carbon  into  the  glaze  at  the  period  of  troubled 
fusion,  a  pinhole  will  be  the  consequence,  after  the  glaze  is 
matured. 

Mr  Plusch :  Do  these  particles  of  carbon  burn 
through  and  produce  pinholes? 

Mr.  Purdy :     Not  in  all  cases. 

Mr.  Plusch  :  I  find,  in  my  experience,  that  carbon 
settling  on  the  green  ware  before  it  is  glazed,  or  settling  on 
the  glaze  before  the  glaze  is  burned,  does  not  produce  pin- 
holing  in  the  firing.  Very  often  particles  fall  on,  after  the 
pieces  are  sprayed,  and  we  take  no  account  of  it. 


FRITTED  GLAZES.  91 

Mr.  Purdy :  Carbon  deposited  either  in  or  on  the 
glaze  before  tiring,  will  as  a  rule  be  burnt  out  before  the 
glaze  has  reached  its  first  boiling  period.  Hence  you  ought 
not  to  expect  it  to  produce  pin-holing  under  those  condi- 
tions. 

Mr.  Gray:  During  the  Boston  meeting  I  brought  up 
the  question  as  to  the  appearance  of  pinholes  in  different 
parts  of  the  kilns  and  at  regular  periods.  I  never  found 
a  probable  solution  until  I  made  some  experiments  last 
fall,  and  I  now  believe  the  trouble  is  wholly  in  the  carbon. 

Mr.  PI  it sch  :  In  my  experience  I  found  that  pin-holing 
occurred  in  the  bottom  of  the  kilns,  especially  when  we 
fired  quickly,  or  in  other  words,  water-smoked  too  rapidly. 
I  have  entirely  overcome  this  by  water-smoking  more  slow- 
ly. Pin-holing  on  glazed  terra  cotta  is  also  produced  by  the 
burning  out  of  finely  divided  carbonaceous  materials  acci- 
dentally introduced  into  the  body,  or  by  the  premature 
fusion  of  low-heat  glaze  materials  introduced  with  the 
saggars  used  for  grog. 

In  both  of  these  cases  cavities  are  produced  on  the 
surface  of  the  ware,  under  the  glaze,  before  it  lias  matured, 
and  show  up  on  the  burned  ware  as  pinholes. 

Mr.  Purdy :  In  other  words,  you  have  overcome  the 
pin-holing  by  permitting  the  carbon  that  is  in  the  glaze  to 
burn  out,  and  by  not  permitting  any  more  to  deposit  on 
the  glaze  after  fusion  has  begun. 

Mr.  Hope :  We  fire  with  gas  and  there  is  no  possible 
chance  of  having  free  carbon  formed,  yet  we  have  pin- 
holing. 

Mr.  Purdy :     You  use  a  fritted  glaze? 

Mr.  Hope :  Yes.  I  put  it  down  to  the  boiling  of  the 
glaze. 

Mr.  Goodwin  :  Did  you  say,  Mr.  Hope,  that  there 
were  no  fumes  from  the  gas? 

Mr.  Hope:  I  meant  that  there  was  no  uncombined 
carbon ;  that  is,  no  free  carbon  in  the  kiln. 

Mr.  Goodwin :  My  experience  goes  to  prove  the  con- 
trary.   I  have  seen  smoke  from  gas,  almost  as  from  coal. 


92  FRITTED  GLAZES. 

Mr.  Hope:  I  accept  the  correction.  I  have  seen  the 
same  thing  myself,  but  failed  to  think  of  it  in  this  connec- 
tion, before. 

Mr.  Goodwin:  I  will  ask  Mr.  Purdy  if  he  has  had 
experience  in  glazes  high  in  A1203,  as  to  the  probability  of 
their  spitting  out? 

Mr.  Purdy :  That  is  a  matter  with  which  I  have  had 
no  experience. 

Mr.  Goodwin :  I  have  found  glazes  of  that  nature  in- 
clined to  spit  out. 

Mr.  Millar:  I  make  enameled  bricks  and  burn  in 
muffle  kilns,  and  use  a  raw  glaze  and  am  not  troubled  much 
with  pin-holing.  But  I  find  occasionally  here  and  there 
throughout  the  kiln,  bricks  which  are  covered  with  pin- 
holes, while  all  around  them,  other  bricks  will  be  perfect. 
I  have  been  trying  to  gather  from  Mr.  Purdy's  paper  and 
remarks,  what  could  be  the  cause  of  that,  but  I  have  not 
been  able  to  do  so. 

Mr.  Purdy :  When  I  was  working  in  the  stoneware 
business,  I  found  if  the  biscuit  ware  was  not  thoroughly 
brushed,  we  would  have  pinholes.  The  dust  is  sometimes 
common  inorganic  dirt  and  oftentimes  it  is  soot  or  carbon. 
If  that  dirt  was  not  brushed  off,  there  would  be  pinholes  as 
a  consequence.  But  when  the  ware  was  thoroughly 
brushed,  it  would  be  free  from  pin-holing,  provided  we  were 
careful  not  to  smoke  the  kiln  in  the  early  stages  of  firing. 

The  Chair:  In  regard  to  this  question  of  pin-holing, 
I  had  the  question  put  to  me  some  time  since  by  a  member, 
how  to  prevent  it.  Not  working  on  Mr.  Purdy's  theory, 
but  rather  on  one  of  my  own  which  I  presented  to  the  so- 
ciety some  two  or  three  years  ago,  I  said  the  way  to  prevent 
it  was  to  be  careful  not  to  cool  the  kiln  too  rapidly.  A  sud- 
den chill  produces  pin-holing,  and  we  have  been  practically 
able  to  eliminate  it  from  our  ware,  merely  by  a  slow  cool- 
ing. At  the  Cleveland  summer  meeting,  while  visiting  a 
stoneware  works  in  Akron  where  they  were  just  firing  off  a 
kiln,  the  old  burner  was  covering  up  every  opening  in  sight. 
I  asked  him  what  he  was  doing?    He  said,  "Why,  if  I  don't 


FRITTED   GLAZES.  93 

get  this  kiln  closed  tight  as  quickly  as  possible,  the  ware 
will  be  all  piiiholed."  I  attribute  it,  as  I  did  in  that  paper, 
to  the  liberation  of  carbonic  acid  gas. 

Mr.  Purdy:  In  parts  of  this  paper  which  were  not 
read,  we  discuss  how  the  probable  reduction  of  lead  in  a 
glaze  makes  the  glaze  more  viscous,  and  that  the  glaze  does 
uot  fuse  as  readily  as  when  free  from  the  carbon,  but  that 
by  continued  soaking  in  the  finishing  heat,  the  normal 
condition  would  be  re-produced,  the  carbon  burned  out,  the 
lead  oxydized,  and  we  would  have  the  glaze  at  its  greatest 
fluidity,  and  consequently  would  not  have  pin-holing.  In 
other  words,  carbon  indirectly  stiffens  the  glaze  and  pre- 
vents the  healing  over  of  these  pinhole  defects. 

Besides  this,  pin-holing  is  probably  not  all  due  to  one 
cause.    There  are  probably  other  causes. 

Mr.  Binns:  I  am  very  glad  we  have  drawn  that 
confession  from  Mr.  Purdy.  I  was  afraid  that  he  was  com- 
mitting himself  to  the  opinion  that  this  was  the  one  and 
only  cause,  whereas  everyone,  I  am  sure,  knows  there  are 
many  causes.  It  is  a  new  thought  to  me,  that  particles  of 
carbon  deposited  on  the  surface  of  the  glaze  can  cause  pin- 
holing,  and  I  was  about  to  make  other  suggestions  which 
Mr.  Purdy's  last  remark  renders  unnecessary.  I  think  it 
must  be  true  that  when  carbon  is  deposited  on  the  glaze  in 
the  early  stages  of  the  burn  it  is  not  deposited  as  particles 
but  as  a  film,  and  I  hardly  sec  how  a  film  of  carbon  should 
break  into  minute  particles  and  cause  pin-holing.  The 
particles  could  not  be  carried  in  through  the  saggars,  and 
the  smoke  deposited  would  be  in  the  nature  of  a  film.  We 
must  be  cautious  in  claiming  as  facts  what  only  appear  as 
phenomena. 

It  is  a  pity  we  could  not  have  seen  the  samples  for  this 
piece  of  work;  apart  from  that,  we  have  here  a  mass  of 
material  which  it  will  take  a  long  time  to  digest.  I  will 
ask  what  means  were  used  to  determine  when  a  glaze  was 
immature  or  when  overtired?  What  is  meant  by  an  "over- 
fired"  glaze?  Is  it  a  glaze  which  has  proven  too  fluid,  and 
has  escaped  control,  or  a  glaze  which  has  partly  volatilized 


94  FRITTED   GLAZE. 

and  lost  its  beauty?  This  must  be  largely  a  matter  of  opin- 
ion. I  do  not  know  of  any  test  by  which  a  man  can  say  a 
glaze  is  immature  or  overtired,  and  I  think  we  ought  to 
have  Mr.  Purdy's  point  of  view  on  that  point. 

Mr.  Purdy :  In  the  case  of  our  glazes,  as  the  heat  in- 
creased, the  glaze  passed  gradually  from  a  porous  to  a 
vitrified  coating,  then  to  that  stage  of  bubbly  fusion,  then 
into  quiet  fusion.  As  soon  as  the  glossiness  of  the  glaze 
became  dim  by  overheating,  or  as  soon  as  the  glaze  began 
to  show  the  second  boiling,  we  called  it  overtired.  We  did 
not,  however,  have  any  dimness  due  to  overfiring  in  our 
cases. 

Mr.  Goodwin  :  Do  you  mean  that  at  the  second  boil- 
ing you  got  pin-holing? 

Mr.  Purdy :  No ;  at  that  stage  it  looked  like  "curdled 
cream." 

Mr.  Goodwin  :  My  experience  has  been  that  you  can 
get  pin-holing  at  the  last  stage  as  well  as  during  the  earlier 
stage,  but  it  is  of  a  different  type. 

Mr.  Purdy :  We  explained  that  in  a  part  of  the  paper 
which  was  not  read,  viz.,  that  the  pinholes  due  to  over- 
firing  seemed  to  go  clear  through  and  were  larger,  while 
the  others,  due  to  the  carbon,  were  smaller,  and  apparently 
only  surface  phenomena. 


FRITTED   GLAZES  87 

as  to  produce  a  cloudy  or  milky  appearance.  When  over- 
tired, the  effect  is  similar  to  the  curdiness  produced  in  a 
glaze  by  addition  of  raw  tin  oxide.  When  fired  to  the  best 
maturity  of  the  glaze,  the  opalescent  effects  produced  by 
supersaturation  with  Si02  are  distinctly  crystalline. 

(5)  While  the  writers  have  advanced  the  opinion 
that  this  opalescent  effect  is  produced  by  supersaturation 
and  separating  out  of  Si02,  they  can  offer  no  evidence  con- 
tradictory to  the  hypothesis  that  the  material  thus  separ- 
ated out  is  a  borate.  High  content  of  B203  is  required,  and 
the  opalescence  here  described  not  only  does  not  appear 
unless  BoO;>  is  proportionately  high,  but  it  increases  in  in- 
tensity and  is  produced  in  wider  and  wider  variation  as 
the  B^Oo  increases  in  proportion  to  Si02.  The  writers  have 
based  their  opinion  that  this  phenomenon  is  due  to  the 
supersaturation  of  the  glaze  by  Si02,  on  the  chemical  com- 
position of  milky  opals  as  they  occur  in  nature. 

ATTACK  ON  THE  BODY. 

The  supposition  has  been  advanced  that  A1203  is  the 
principal  constituent  taken  from  the  body  by  the  glaze. 
The  facts  that  support  this  assumption  are  as  follows: 

(1)  The  glazes  lowest  in  A1203  (0.10—0.20  Eqv.) 
show  this  phenomenon  to  the  greatest  extent. 

(2)  The  glazes  lowest  in  A1203  show  the  fine-mesh 
crazing  which  results  from  incorporating  constituents 
from  the  body  into  the  glaze  layer  that  is  contiguous  to  the 
body,  irrespective  of  oxygen  ratio.  It  is  a  fact,  however, 
that  fine-mesh  crazing  decreases  as  the  oxygen  ratio  in- 
creases, but  this  is  attributed  to  the  approximate  satura- 
tion of  the  matrix  with  acid,  as  is  evidenced  in  the  appear- 
ance of  opalescence  as  early  as  Group  VI. 

(3)  Glazes  having  a  low  oxygen  ratio,  and  low  A120.5 
content,  decrease  in  fine-mesh  crazing  as  the  intensity  of 
heat  increases. 

(4)  Glazes  that  craze  in  fine  meshes  when  thick  are 
crazed  only  in  hair  lines  when  thin. 

P.  A  F.— 7. 


88  FRITTED   GLAZES. 

(5)  Increase  of  originally  added  AL03  decreases 
fine-mesh  crazing  under  less  intense  temperatures,  but  in- 
creases it  as  the  temperature  becomes  more  intense. 

(6)  Fine-mesh  crazing  is  certainly  due  to  lack  of 
homogeniety  in  the  composition  of  the  upper  and  lower  sur- 
face of  the  glaze  layer,  and  this  lack  of  homogeniety  is 
more  pronounced,  the  more  viscous  the  glaze  is  rendered 
by  the  incorporated  constituent. 

(7)  Pine-mesh  crazing  developed  at  temperatures 
below  Cone  5,  can  be  counteracted  by  increasing  the  A1203 
content  of  the  glaze.  If  it  is  developed  at  higher  heats,  it 
can  be  counteracted  by  decreasing  the  A1203  content,  es- 
pecially when  the  oxygen  ratio  is  low. 

PIN-HOLING. 

Pin-holing  and  blistering  has  been  proven  in  these 
studies  to  be  primarily  due  to  the  combustion  of  carbon 
that  had  .been  entrapped  during  the  fritting  stage  of  glaze 
formation.  The  glazes  that  come  to  a  quiet  fusion  earliest 
are  less  likely  to  exhibit  this  defect.  Raw  feldspar  glazes 
show  this  defect  more  than  the  raw  Cornwall  stone  glazes, 
but  as  was  shown  by  Mr.  Coulter1  the  stone  glazes  are 
more  fusible,  and  have  a  longer  heat  range,  and  therefore 
are  subjected  for  a  shorter  time  to  the  influence  of  carbon 
in  the  fire  gases  before  quiet  fusion  sets  in.  As  the  evidence 
in  this  study  in  the  case  of  fritted  glazes  agrees  with  simi- 
lar phenomena  observed  in  raw  lead  glazes,  there  seems 
to  be  no  doubt  that  the  statements  in  regard  to  the  cause 
of  pin-holing  given  above  are  correct. 

In  conclusion  the  writers  wish  to  express  their  appre- 
ciation of  the  hearty  support  and  substantial  encourage- 
ment they  received  from  the  authorities  of  the  University 
of  Illinois  in  general,  and  from  Professor  C.  W.  Eolfe,  Di- 
rector of  the  Ceramic  Department,  in  particular.  Had 
they  not  granted  every  facility  within  their  power,  the  exe- 


'Trans.  Am.  Cer.  Soc.  Vol.  VII,  p.  356. 


n:i  ITKI)  QOjAZBB.  89 

cution  of  this  study  in  its  details  would  have  been  impos- 
sible. We  wish  also  to  express  our  further  appreciation  of 
the  privilege  of  offering  the  results  of  this  research  to  the 
American  Ceramic  Society  previous  to  its  issuance  as  a 
University  Bulletin. 

DISCUSSION. 

The  Chair:  By  the  presentation  of  this  very  able 
paper,  I  am  impressed  at  the  outset  with  the  growing  im- 
portance of  making  provision,  as  soon  as  our  financial  con- 
dition shall  warrant  it,  for  the  publication  of  such  papers 
prior  to  our  meetings,  so  that  we  may  study  them,  and  be 
better  able  to  discuss  them  when  we  come  to  the  meeting. 
We  have  realized  this  necessity  during  past  sessions,  but 
the  need  is  growing  more  apparent  all  the  time.  Still  this 
paper  contains  so  many  interesting  points  that  we  ought 
to  have  a  general  discussion  on  it. 

Mr.  Parmelee:  I  feel,  of  course,  wholly  unable  to 
discuss  the  detail  of  this  work.  It  certainly  is  a  big  sub- 
ject, and  I  think  brings  pleasure  to  all  members  to  see  such 
a  substantial  contribution  on  it.  Did  I  understand  you  to 
say,  Mr.  Fox,  that  coke  was  used  as  a  fuel  ? 

Mr.  Fox:     Yes,  sir. 

Mr.  Parmelee:  Then  how  do  you  attribute  the  pin- 
holing  to  carbon  deposited  on  the  glaze?  In  what  form  is 
it  deposited  on  the  glaze? 

Mr.  Purdy.  Coke  certainly  contains  carbon,  which 
on  combustion  is  driven  off.  Not  all  the  carbon,  however, 
is  oxydised  to  either  carbon  monoxide  or  carbon  dioxide; 
some  of  it  may  be  carried  by  the  draft  as  particles  of  free 
carbon. 

Mr.  Parmelee:  Then  these  are  particles  of  fuel,  car- 
ried mechanically  and  deposited  mechanically? 

Mr.  Purdy:  We  know  that  black  smoke  contains 
about  two  percent,  of  carbon,  and  when  the  coke  is  freshly 
thrown  on  the  grate  bars  there  will  be  an  evolution  of 
smoke  or  black  gases,  and  that  is  sufficient  to  cause  a  de- 


90  FRITTED  GLAZES. 

posit  of  carbon  on  the  glaze,  notwithstanding  the  fact  that 
the  saggers  are  thoroughly  sealed. 

The  Chair :  You  assume  that  in  all  of  the  kiln  firings 
you  formed  this  deposit,  but  in  some  were  able  to  free  the 
ware  from  it  more  completely  than  in  others? 

Mr.  Purdy :  No.  When  deposited  prior  to  fusion, 
then  the  carbon  would  be  incorporated  and  cause  pin- 
holing  ;  but  when  deposited  after  the  quiet  stages  of  fusion 
has  been  reached,  the  glaze  would  not  then  be  affected. 

The  Chair :  But  all  appear  to  have  the  deposit  in  the 
earlier  stage,  but  some  are  able  to  free  themselves? 

Mr.  Purdy :  No.  Carbon  is  effective  in  producing  pin- 
holes only  when  it  becomes  incorporated  into  the  glaze.  If 
carbon  is  deposited  on  the  glaze  during  the  boiling  period, 
it  will  become  incorporated.  A  glaze  which  has  its  initial 
boiling  period  early,  or  which  has  a  very  short  or  mild 
boiling  period,  will  not  be  affected  as  much  as  the  glaze 
that  boils  later  and  consequently  harder. 

The  Chair:  I  will  ask  Mr.  Purdy  what  method  he 
used  to  determine  over-firing? 

Mr.  Purdy:  We  determined  that  by  its  appearance, 
i.  e.  when  it  began  to  pass  into  the  second  boiling  stage.  I 
do  not  know  why  it  should  boil  the  second  time,  but  it  does. 

Mr.  Par  melee:  I  have  never  had  any  experience  in 
coke  firing,  but  am  surprised  to  learn  that  in  burning  coke 
you  have  black  smoke. 

Mr.  Purdy:  You  will  have  some;  and  if  you  put  one 
speck  of  carbon  into  the  glaze  at  the  period  of  troubled 
fusion,  a  pinhole  will  be  the  consequence,  after  the  glaze  is 
matured. 

Mr  Plusch :  Do  these  particles  of  carbon  burn 
through  and  produce  pinholes? 

Mr.  Purdy :     Not  in  all  cases. 

Mr.  Plusch :  I  find,  in  my  experience,  that  carbon 
settling  on  the  green  ware  before  it  is  glazed,  or  settling  on 
the  glaze  before  the  glaze  is  burned,  does  not  produce  pin- 
holing  in  the  firing.  Very  often  particles  fall  on,  after  the 
pieces  are  sprayed,  and  we  take  no  account  of  it. 


FRITTED  GLAZES.  91 

Mr.  Purdy :  Carbon  deposited  either  in  or  on  the 
glaze  before  firing,  will  as  a  rule  be  burnt  out  before  the 
glaze  has  reached  its  first  boiling  period.  Hence  you  ought 
not  to  expect  it  to  produce  pin-holing  under  those  condi- 
tions. 

Mr.  Gray :  During  the  Boston  meeting  I  brought  up 
the  question  as  to  the  appearance  of  pinholes  in  different 
parts  of  the  kilns  and  at  regular  periods.  I  never  found 
a  probable  solution  until  I  made  some  experiments  last 
fall,  and  I  now  believe  the  trouble  is  wholly  in  the  carbon. 

Mr.  Plusch :  In  my  experience  I  found  that  pin-holing 
occurred  in  the  bottom  of  the  kilns,  especially  when  we 
fired  quickly,  or  in  other  words,  water-smoked  too  rapidly. 
I  have  entirely  overcome  this  by  water-smoking  more  slow- 
ly. Pin-holing  on  glazed  terra  cotta  is  also  produced  by  the 
burning  out  of  finely  divided  carbonaceous  materials  acci- 
dentally introduced  into  the  body,  or  by  the  premature 
fusion  of  low-heat  glaze  materials  introduced  with  the 
saggars  used  for  grog. 

In  both  of  these  cases  cavities  are  produced  on  the 
surface  of  the  ware,  under  the  glaze,  before  it  has  matured, 
and  show  up  on  the  burned  ware  as  pinholes. 

Mr.  Purdy:  In  other  words,  you  have  overcome  the 
pin-holing  by  permitting  the  carbon  that  is  in  the  glaze  to 
burn  out,  and  by  not  permitting  any  more  to  deposit  on 
the  glaze  after  fusion  has  begun. 

Mr.  Hope :  We  fire  with  gas  and  there  is  no  possible 
chance  of  having  free  carbon  formed,  yet  we  have  pin- 
holing. 

Mr.  Purdy :     You  use  a  fritted  glaze? 

Mr.  Hope :  Yes.  I  put  it  down  to  the  boiling  of  the 
glaze. 

Mr.  Goodwin:  Did  you  say,  Mr.  Hope,  that  there 
were  no  fumes  from  the  gas? 

Mr.  Hope:  I  meant  that  there  was  no  uncombined 
carbon ;  that  is,  no  free  carbon  in  the  kiln. 

Mr.  Goodicin :  My  experience  goes  to  prove  the  con- 
trary.   I  have  seen  smoke  from  gas,  almost  as  from  coal. 


92  FRITTED   GLAZES. 

Mr.  Eope:  I  accept  the  correction.  I  have  seen  the 
same  thing  myself,  but  failed  to  think  of  it  in  this  connec- 
tion, before. 

Mr.  Goodicin:  I  will  ask  Mr.  Purdy  if  he  has  had 
experience  in  glazes  high  in  A1203,  as  to  the  probability  of 
their  spitting  out? 

Mr.  Purdy :  That  is  a  matter  with  which  I  have  had 
no  experience. 

Mr.  Goodwin :  I  have  found  glazes  of  that  nature  in- 
clined to  spit  out. 

Mr.  Millar:  I  make  enameled  bricks  and  burn  in 
muffle  kilns,  and  use  a  raw  glaze  and  am  not  troubled  much 
with  pin-holing.  But  I  find  occasionally  here  and  there 
throughout  the  kiln,  bricks  which  are  covered  with  pin- 
holes, while  all  around  them,  other  bricks  will  be  perfect. 
I  have  been  trying  to  gather  from  Mr.  Purdy's  paper  and 
remarks,  what  could  be  the  cause  of  that,  but  I  have  not 
been  able  to  do  so. 

Mr.  Purdy:  When  I  was  working  in  the  stoneware 
business,  I  found  if  the  biscuit  ware  was  not  thoroughly 
brushed,  we  would  have  pinholes.  The  dust  is  sometimes 
common  inorganic  dirt  and  oftentimes  it  is  soot  or  carbon. 
If  that  dirt  was  not  brushed  off,  there  would  be  pinholes  as 
a  consequence.  But  when  the  ware  was  thoroughly 
brushed,  it  would  be  free  from  pin-holing,  provided  we  were 
careful  not  to  smoke  the  kiln  in  the  early  stages  of  firing. 

The  Chair:  In  regard  to  this  question  of  pin-holing, 
I  had  the  question  put  to  me  some  time  since  by  a  member, 
how  to  prevent  it.  Not  working  on  Mr.  Purdy's  theory, 
but  rather  on  one  of  my  own  which  I  presented  to  the  so- 
ciety some  two  or  three  years  ago,  I  said  the  way  to  prevent 
it  was  to  be  careful  not  to  cool  the  kiln  too  rapidly.  A  sud- 
den chill  produces  pin-holing,  and  we  have  been  practically 
able  to  eliminate  it  from  our  ware,  merely  by  a  slow  cool- 
ing. At  the  Cleveland  summer  meeting,  while  visiting  a 
stoneware  works  in  Akron  where  they  were  just  firing  off  a 
kiln,  the  old  burner  was  covering  up  every  opening  in  sight. 
I  asked  him  what  he  was  doing?    He  said,  "Why,  if  I  don't 


FRITTED   GLAZES.  OS 

get  this  kiln  closed  tight  as  quickly  as  possible,  the  ware 
will  be  all  piiiholed."  I  attribute  it,  as  I  did  in  that  paper, 
to  the  liberation  of  carbonic  acid  gas. 

Mr.  Purdy:  In  parts  of  this  paper  which  were  not 
read,  we  discuss  how  the  probable  reduction  of  lead  in  a 
glaze  makes  the  glaze  more  viscous,  and  that  the  glaze  does 
not  fuse  as  readily  as  when  free  from  the  carbon,  but  that 
by  continued  soaking  in  the  finishing  heat,  the  normal 
condition  would  be  re-produced,  the  carbon  burned  out,  the 
lead  oxydized,  and  we  would  have  the  glaze  at  its  greatest 
fluidity,  and  consequently  would  not  have  pin-holing.  In 
other  words,  carbon  indirectly  stiffens  the  glaze  and  pre- 
vents the  healing  over  of  these  pinhole  defects. 

Besides  this,  pin-holing  is  probably  not  all  due  to  one 
cause.    There  are  probably  other  causes. 

Mr.  Binns:  I  am  very  glad  we  have  drawn  thai 
confession  from  Mr.  Purdy.  I  was  afraid  that  he  was  com- 
mitting himself  to  the  opinion  that  this  was  the  one  and 
only  cause,  whereas  everyone,  I  ani  sure,  knows  there  are 
many  causes.  It  is  a  new  thought  to  me,  that  particles  of 
carbon  deposited  on  the  surface  of  the  glaze  can  cause  pin- 
holing,  and  I  was  about  to  make  other  suggestions  which 
Mr.  Purdy's  last  remark  renders  unnecessary.  I  think  it 
must  be  true  that  when  carbon  is  deposited  on  the  glaze  in 
the  early  stages  of  the  burn  it  is  not  deposited  as  particles 
but  as  a  film,  and  I  hardly  see  how  a  film  of  carbon  should 
break  into  minute  particles  and  cause  pin-holing.  The 
particles  could  not  be  carried  in  through  the  saggars,  and 
the  smoke  deposited  would  be  in  the  nature  of  a  film.  We 
must  be  cautious  in  claiming  as  facts  what  only  appear  as 
phenomena. 

It  is  a  pity  we  could  not  have  seen  the  samples  for  this 
piece  of  work;  apart  from  that,  we  have  here  a  mass  of 
material  which  it  will  take  a  long  time  to  digest.  T  will 
ask  what  means  were  used  to  determine  when  a  glaze  was 
immature  or  when  overtired?  What  is  meant  by  an  "over- 
fired"  glaze?  Is  it  a  glaze  which  has  proven  too  fluid,  and 
has  escaped  control,  or  a  glaze  which  has  partly  volatilized 


94  FRITTED   GLAZE. 

and  lost  its  beauty?  This  must  be  largely  a  matter  of  opin- 
ion. I  do  not  know  of  any  test  by  which  a  man  can  say  a 
glaze  is  immature  or  overtired,  and  I  think  we  ought  to 
have  Mr.  Purdy's  point  of  view  on  that  point. 

Mr.  Purdy :  In  the  case  of  our  glazes,  as  the  heat  in- 
creased, the  glaze  passed  gradually  from  a  porous  to  a 
vitrified  coating,  then  to  that  stage  of  bubbly  fusion,  then 
into  quiet  fusion.  As  soon  as  the  glossiness  of  the  glaze 
became  dim  by  overheating,  or  as  soon  as  the  glaze  began 
to  show  the  second  boiling,  we  called  it  overtired.  We  did 
not,  however,  have  any  dimness  due  to  overfiring  in  our 
cases. 

Mr.  Goodwin :  Do  you  mean  that  at  the  second  boil- 
ing you  got  pin-holing? 

Mr.  Purdy :  No ;  at  that  stage  it  looked  like  "curdled 
cream." 

Mr.  Goodwin :  My  experience  has  been  that  you  can 
get  pin-holing  at  the  last  stage  as  well  as  during  the  earlier 
stage,  but  it  is  of  a  different  type. 

Mr.  Purdy :  We  explained  that  in  a  part  of  the  paper 
which  was  not  read,  viz.,  that  the  pinholes  due  to  over- 
firing  seemed  to  go  clear  through  and  were  larger,  while 
the  others,  due  to  the  carbon,  were  smaller,  and  apparently 
only  surface  phenomena. 


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